[PATCH] sem2mutex: mm/slab.c

[safe/jmp/linux-2.6] / mm / shmem.c

/*
 * Resizable virtual memory filesystem for Linux.
 *
 * Copyright (C) 2000 Linus Torvalds.
 *               2000 Transmeta Corp.
 *               2000-2001 Christoph Rohland
 *               2000-2001 SAP AG
 *               2002 Red Hat Inc.
 * Copyright (C) 2002-2005 Hugh Dickins.
 * Copyright (C) 2002-2005 VERITAS Software Corporation.
 * Copyright (C) 2004 Andi Kleen, SuSE Labs
 *
 * Extended attribute support for tmpfs:
 * Copyright (c) 2004, Luke Kenneth Casson Leighton <lkcl@lkcl.net>
 * Copyright (c) 2004 Red Hat, Inc., James Morris <jmorris@redhat.com>
 *
 * This file is released under the GPL.
 */

/*
 * This virtual memory filesystem is heavily based on the ramfs. It
 * extends ramfs by the ability to use swap and honor resource limits
 * which makes it a completely usable filesystem.
 */

#include <linux/config.h>
#include <linux/module.h>
#include <linux/init.h>
#include <linux/devfs_fs_kernel.h>
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/mman.h>
#include <linux/file.h>
#include <linux/swap.h>
#include <linux/pagemap.h>
#include <linux/string.h>
#include <linux/slab.h>
#include <linux/backing-dev.h>
#include <linux/shmem_fs.h>
#include <linux/mount.h>
#include <linux/writeback.h>
#include <linux/vfs.h>
#include <linux/blkdev.h>
#include <linux/security.h>
#include <linux/swapops.h>
#include <linux/mempolicy.h>
#include <linux/namei.h>
#include <asm/uaccess.h>
#include <asm/div64.h>
#include <asm/pgtable.h>

/* This magic number is used in glibc for posix shared memory */
#define TMPFS_MAGIC     0x01021994

#define ENTRIES_PER_PAGE (PAGE_CACHE_SIZE/sizeof(unsigned long))
#define ENTRIES_PER_PAGEPAGE (ENTRIES_PER_PAGE*ENTRIES_PER_PAGE)
#define BLOCKS_PER_PAGE  (PAGE_CACHE_SIZE/512)

#define SHMEM_MAX_INDEX  (SHMEM_NR_DIRECT + (ENTRIES_PER_PAGEPAGE/2) * (ENTRIES_PER_PAGE+1))
#define SHMEM_MAX_BYTES  ((unsigned long long)SHMEM_MAX_INDEX << PAGE_CACHE_SHIFT)

#define VM_ACCT(size)    (PAGE_CACHE_ALIGN(size) >> PAGE_SHIFT)

/* info->flags needs VM_flags to handle pagein/truncate races efficiently */
#define SHMEM_PAGEIN     VM_READ
#define SHMEM_TRUNCATE   VM_WRITE

/* Definition to limit shmem_truncate's steps between cond_rescheds */
#define LATENCY_LIMIT    64

/* Pretend that each entry is of this size in directory's i_size */
#define BOGO_DIRENT_SIZE 20

/* Flag allocation requirements to shmem_getpage and shmem_swp_alloc */
enum sgp_type {
        SGP_QUICK,      /* don't try more than file page cache lookup */
        SGP_READ,       /* don't exceed i_size, don't allocate page */
        SGP_CACHE,      /* don't exceed i_size, may allocate page */
        SGP_WRITE,      /* may exceed i_size, may allocate page */
};

static int shmem_getpage(struct inode *inode, unsigned long idx,
                         struct page **pagep, enum sgp_type sgp, int *type);

static inline struct page *shmem_dir_alloc(gfp_t gfp_mask)
{
        /*
         * The above definition of ENTRIES_PER_PAGE, and the use of
         * BLOCKS_PER_PAGE on indirect pages, assume PAGE_CACHE_SIZE:
         * might be reconsidered if it ever diverges from PAGE_SIZE.
         */
        return alloc_pages(gfp_mask, PAGE_CACHE_SHIFT-PAGE_SHIFT);
}

static inline void shmem_dir_free(struct page *page)
{
        __free_pages(page, PAGE_CACHE_SHIFT-PAGE_SHIFT);
}

static struct page **shmem_dir_map(struct page *page)
{
        return (struct page **)kmap_atomic(page, KM_USER0);
}

static inline void shmem_dir_unmap(struct page **dir)
{
        kunmap_atomic(dir, KM_USER0);
}

static swp_entry_t *shmem_swp_map(struct page *page)
{
        return (swp_entry_t *)kmap_atomic(page, KM_USER1);
}

static inline void shmem_swp_balance_unmap(void)
{
        /*
         * When passing a pointer to an i_direct entry, to code which
         * also handles indirect entries and so will shmem_swp_unmap,
         * we must arrange for the preempt count to remain in balance.
         * What kmap_atomic of a lowmem page does depends on config
         * and architecture, so pretend to kmap_atomic some lowmem page.
         */
        (void) kmap_atomic(ZERO_PAGE(0), KM_USER1);
}

static inline void shmem_swp_unmap(swp_entry_t *entry)
{
        kunmap_atomic(entry, KM_USER1);
}

static inline struct shmem_sb_info *SHMEM_SB(struct super_block *sb)
{
        return sb->s_fs_info;
}

/*
 * shmem_file_setup pre-accounts the whole fixed size of a VM object,
 * for shared memory and for shared anonymous (/dev/zero) mappings
 * (unless MAP_NORESERVE and sysctl_overcommit_memory <= 1),
 * consistent with the pre-accounting of private mappings ...
 */
static inline int shmem_acct_size(unsigned long flags, loff_t size)
{
        return (flags & VM_ACCOUNT)?
                security_vm_enough_memory(VM_ACCT(size)): 0;
}

static inline void shmem_unacct_size(unsigned long flags, loff_t size)
{
        if (flags & VM_ACCOUNT)
                vm_unacct_memory(VM_ACCT(size));
}

/*
 * ... whereas tmpfs objects are accounted incrementally as
 * pages are allocated, in order to allow huge sparse files.
 * shmem_getpage reports shmem_acct_block failure as -ENOSPC not -ENOMEM,
 * so that a failure on a sparse tmpfs mapping will give SIGBUS not OOM.
 */
static inline int shmem_acct_block(unsigned long flags)
{
        return (flags & VM_ACCOUNT)?
                0: security_vm_enough_memory(VM_ACCT(PAGE_CACHE_SIZE));
}

static inline void shmem_unacct_blocks(unsigned long flags, long pages)
{
        if (!(flags & VM_ACCOUNT))
                vm_unacct_memory(pages * VM_ACCT(PAGE_CACHE_SIZE));
}

static struct super_operations shmem_ops;
static struct address_space_operations shmem_aops;
static struct file_operations shmem_file_operations;
static struct inode_operations shmem_inode_operations;
static struct inode_operations shmem_dir_inode_operations;
static struct vm_operations_struct shmem_vm_ops;

static struct backing_dev_info shmem_backing_dev_info  __read_mostly = {
        .ra_pages       = 0,    /* No readahead */
        .capabilities   = BDI_CAP_NO_ACCT_DIRTY | BDI_CAP_NO_WRITEBACK,
        .unplug_io_fn   = default_unplug_io_fn,
};

static LIST_HEAD(shmem_swaplist);
static DEFINE_SPINLOCK(shmem_swaplist_lock);

static void shmem_free_blocks(struct inode *inode, long pages)
{
        struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
        if (sbinfo->max_blocks) {
                spin_lock(&sbinfo->stat_lock);
                sbinfo->free_blocks += pages;
                inode->i_blocks -= pages*BLOCKS_PER_PAGE;
                spin_unlock(&sbinfo->stat_lock);
        }
}

/*
 * shmem_recalc_inode - recalculate the size of an inode
 *
 * @inode: inode to recalc
 *
 * We have to calculate the free blocks since the mm can drop
 * undirtied hole pages behind our back.
 *
 * But normally   info->alloced == inode->i_mapping->nrpages + info->swapped
 * So mm freed is info->alloced - (inode->i_mapping->nrpages + info->swapped)
 *
 * It has to be called with the spinlock held.
 */
static void shmem_recalc_inode(struct inode *inode)
{
        struct shmem_inode_info *info = SHMEM_I(inode);
        long freed;

        freed = info->alloced - info->swapped - inode->i_mapping->nrpages;
        if (freed > 0) {
                info->alloced -= freed;
                shmem_unacct_blocks(info->flags, freed);
                shmem_free_blocks(inode, freed);
        }
}

/*
 * shmem_swp_entry - find the swap vector position in the info structure
 *
 * @info:  info structure for the inode
 * @index: index of the page to find
 * @page:  optional page to add to the structure. Has to be preset to
 *         all zeros
 *
 * If there is no space allocated yet it will return NULL when
 * page is NULL, else it will use the page for the needed block,
 * setting it to NULL on return to indicate that it has been used.
 *
 * The swap vector is organized the following way:
 *
 * There are SHMEM_NR_DIRECT entries directly stored in the
 * shmem_inode_info structure. So small files do not need an addional
 * allocation.
 *
 * For pages with index > SHMEM_NR_DIRECT there is the pointer
 * i_indirect which points to a page which holds in the first half
 * doubly indirect blocks, in the second half triple indirect blocks:
 *
 * For an artificial ENTRIES_PER_PAGE = 4 this would lead to the
 * following layout (for SHMEM_NR_DIRECT == 16):
 *
 * i_indirect -> dir --> 16-19
 *            |      +-> 20-23
 *            |
 *            +-->dir2 --> 24-27
 *            |        +-> 28-31
 *            |        +-> 32-35
 *            |        +-> 36-39
 *            |
 *            +-->dir3 --> 40-43
 *                     +-> 44-47
 *                     +-> 48-51
 *                     +-> 52-55
 */
static swp_entry_t *shmem_swp_entry(struct shmem_inode_info *info, unsigned long index, struct page **page)
{
        unsigned long offset;
        struct page **dir;
        struct page *subdir;

        if (index < SHMEM_NR_DIRECT) {
                shmem_swp_balance_unmap();
                return info->i_direct+index;
        }
        if (!info->i_indirect) {
                if (page) {
                        info->i_indirect = *page;
                        *page = NULL;
                }
                return NULL;                    /* need another page */
        }

        index -= SHMEM_NR_DIRECT;
        offset = index % ENTRIES_PER_PAGE;
        index /= ENTRIES_PER_PAGE;
        dir = shmem_dir_map(info->i_indirect);

        if (index >= ENTRIES_PER_PAGE/2) {
                index -= ENTRIES_PER_PAGE/2;
                dir += ENTRIES_PER_PAGE/2 + index/ENTRIES_PER_PAGE;
                index %= ENTRIES_PER_PAGE;
                subdir = *dir;
                if (!subdir) {
                        if (page) {
                                *dir = *page;
                                *page = NULL;
                        }
                        shmem_dir_unmap(dir);
                        return NULL;            /* need another page */
                }
                shmem_dir_unmap(dir);
                dir = shmem_dir_map(subdir);
        }

        dir += index;
        subdir = *dir;
        if (!subdir) {
                if (!page || !(subdir = *page)) {
                        shmem_dir_unmap(dir);
                        return NULL;            /* need a page */
                }
                *dir = subdir;
                *page = NULL;
        }
        shmem_dir_unmap(dir);
        return shmem_swp_map(subdir) + offset;
}

static void shmem_swp_set(struct shmem_inode_info *info, swp_entry_t *entry, unsigned long value)
{
        long incdec = value? 1: -1;

        entry->val = value;
        info->swapped += incdec;
        if ((unsigned long)(entry - info->i_direct) >= SHMEM_NR_DIRECT) {
                struct page *page = kmap_atomic_to_page(entry);
                set_page_private(page, page_private(page) + incdec);
        }
}

/*
 * shmem_swp_alloc - get the position of the swap entry for the page.
 *                   If it does not exist allocate the entry.
 *
 * @info:       info structure for the inode
 * @index:      index of the page to find
 * @sgp:        check and recheck i_size? skip allocation?
 */
static swp_entry_t *shmem_swp_alloc(struct shmem_inode_info *info, unsigned long index, enum sgp_type sgp)
{
        struct inode *inode = &info->vfs_inode;
        struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
        struct page *page = NULL;
        swp_entry_t *entry;

        if (sgp != SGP_WRITE &&
            ((loff_t) index << PAGE_CACHE_SHIFT) >= i_size_read(inode))
                return ERR_PTR(-EINVAL);

        while (!(entry = shmem_swp_entry(info, index, &page))) {
                if (sgp == SGP_READ)
                        return shmem_swp_map(ZERO_PAGE(0));
                /*
                 * Test free_blocks against 1 not 0, since we have 1 data
                 * page (and perhaps indirect index pages) yet to allocate:
                 * a waste to allocate index if we cannot allocate data.
                 */
                if (sbinfo->max_blocks) {
                        spin_lock(&sbinfo->stat_lock);
                        if (sbinfo->free_blocks <= 1) {
                                spin_unlock(&sbinfo->stat_lock);
                                return ERR_PTR(-ENOSPC);
                        }
                        sbinfo->free_blocks--;
                        inode->i_blocks += BLOCKS_PER_PAGE;
                        spin_unlock(&sbinfo->stat_lock);
                }

                spin_unlock(&info->lock);
                page = shmem_dir_alloc(mapping_gfp_mask(inode->i_mapping) | __GFP_ZERO);
                if (page)
                        set_page_private(page, 0);
                spin_lock(&info->lock);

                if (!page) {
                        shmem_free_blocks(inode, 1);
                        return ERR_PTR(-ENOMEM);
                }
                if (sgp != SGP_WRITE &&
                    ((loff_t) index << PAGE_CACHE_SHIFT) >= i_size_read(inode)) {
                        entry = ERR_PTR(-EINVAL);
                        break;
                }
                if (info->next_index <= index)
                        info->next_index = index + 1;
        }
        if (page) {
                /* another task gave its page, or truncated the file */
                shmem_free_blocks(inode, 1);
                shmem_dir_free(page);
        }
        if (info->next_index <= index && !IS_ERR(entry))
                info->next_index = index + 1;
        return entry;
}

/*
 * shmem_free_swp - free some swap entries in a directory
 *
 * @dir:   pointer to the directory
 * @edir:  pointer after last entry of the directory
 */
static int shmem_free_swp(swp_entry_t *dir, swp_entry_t *edir)
{
        swp_entry_t *ptr;
        int freed = 0;

        for (ptr = dir; ptr < edir; ptr++) {
                if (ptr->val) {
                        free_swap_and_cache(*ptr);
                        *ptr = (swp_entry_t){0};
                        freed++;
                }
        }
        return freed;
}

static int shmem_map_and_free_swp(struct page *subdir,
                int offset, int limit, struct page ***dir)
{
        swp_entry_t *ptr;
        int freed = 0;

        ptr = shmem_swp_map(subdir);
        for (; offset < limit; offset += LATENCY_LIMIT) {
                int size = limit - offset;
                if (size > LATENCY_LIMIT)
                        size = LATENCY_LIMIT;
                freed += shmem_free_swp(ptr+offset, ptr+offset+size);
                if (need_resched()) {
                        shmem_swp_unmap(ptr);
                        if (*dir) {
                                shmem_dir_unmap(*dir);
                                *dir = NULL;
                        }
                        cond_resched();
                        ptr = shmem_swp_map(subdir);
                }
        }
        shmem_swp_unmap(ptr);
        return freed;
}

static void shmem_free_pages(struct list_head *next)
{
        struct page *page;
        int freed = 0;

        do {
                page = container_of(next, struct page, lru);
                next = next->next;
                shmem_dir_free(page);
                freed++;
                if (freed >= LATENCY_LIMIT) {
                        cond_resched();
                        freed = 0;
                }
        } while (next);
}

static void shmem_truncate_range(struct inode *inode, loff_t start, loff_t end)
{
        struct shmem_inode_info *info = SHMEM_I(inode);
        unsigned long idx;
        unsigned long size;
        unsigned long limit;
        unsigned long stage;
        unsigned long diroff;
        struct page **dir;
        struct page *topdir;
        struct page *middir;
        struct page *subdir;
        swp_entry_t *ptr;
        LIST_HEAD(pages_to_free);
        long nr_pages_to_free = 0;
        long nr_swaps_freed = 0;
        int offset;
        int freed;
        int punch_hole = 0;

        inode->i_ctime = inode->i_mtime = CURRENT_TIME;
        idx = (start + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
        if (idx >= info->next_index)
                return;

        spin_lock(&info->lock);
        info->flags |= SHMEM_TRUNCATE;
        if (likely(end == (loff_t) -1)) {
                limit = info->next_index;
                info->next_index = idx;
        } else {
                limit = (end + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
                if (limit > info->next_index)
                        limit = info->next_index;
                punch_hole = 1;
        }

        topdir = info->i_indirect;
        if (topdir && idx <= SHMEM_NR_DIRECT && !punch_hole) {
                info->i_indirect = NULL;
                nr_pages_to_free++;
                list_add(&topdir->lru, &pages_to_free);
        }
        spin_unlock(&info->lock);

        if (info->swapped && idx < SHMEM_NR_DIRECT) {
                ptr = info->i_direct;
                size = limit;
                if (size > SHMEM_NR_DIRECT)
                        size = SHMEM_NR_DIRECT;
                nr_swaps_freed = shmem_free_swp(ptr+idx, ptr+size);
        }
        if (!topdir)
                goto done2;

        BUG_ON(limit <= SHMEM_NR_DIRECT);
        limit -= SHMEM_NR_DIRECT;
        idx = (idx > SHMEM_NR_DIRECT)? (idx - SHMEM_NR_DIRECT): 0;
        offset = idx % ENTRIES_PER_PAGE;
        idx -= offset;

        dir = shmem_dir_map(topdir);
        stage = ENTRIES_PER_PAGEPAGE/2;
        if (idx < ENTRIES_PER_PAGEPAGE/2) {
                middir = topdir;
                diroff = idx/ENTRIES_PER_PAGE;
        } else {
                dir += ENTRIES_PER_PAGE/2;
                dir += (idx - ENTRIES_PER_PAGEPAGE/2)/ENTRIES_PER_PAGEPAGE;
                while (stage <= idx)
                        stage += ENTRIES_PER_PAGEPAGE;
                middir = *dir;
                if (*dir) {
                        diroff = ((idx - ENTRIES_PER_PAGEPAGE/2) %
                                ENTRIES_PER_PAGEPAGE) / ENTRIES_PER_PAGE;
                        if (!diroff && !offset) {
                                *dir = NULL;
                                nr_pages_to_free++;
                                list_add(&middir->lru, &pages_to_free);
                        }
                        shmem_dir_unmap(dir);
                        dir = shmem_dir_map(middir);
                } else {
                        diroff = 0;
                        offset = 0;
                        idx = stage;
                }
        }

        for (; idx < limit; idx += ENTRIES_PER_PAGE, diroff++) {
                if (unlikely(idx == stage)) {
                        shmem_dir_unmap(dir);
                        dir = shmem_dir_map(topdir) +
                            ENTRIES_PER_PAGE/2 + idx/ENTRIES_PER_PAGEPAGE;
                        while (!*dir) {
                                dir++;
                                idx += ENTRIES_PER_PAGEPAGE;
                                if (idx >= limit)
                                        goto done1;
                        }
                        stage = idx + ENTRIES_PER_PAGEPAGE;
                        middir = *dir;
                        *dir = NULL;
                        nr_pages_to_free++;
                        list_add(&middir->lru, &pages_to_free);
                        shmem_dir_unmap(dir);
                        cond_resched();
                        dir = shmem_dir_map(middir);
                        diroff = 0;
                }
                subdir = dir[diroff];
                if (subdir && page_private(subdir)) {
                        size = limit - idx;
                        if (size > ENTRIES_PER_PAGE)
                                size = ENTRIES_PER_PAGE;
                        freed = shmem_map_and_free_swp(subdir,
                                                offset, size, &dir);
                        if (!dir)
                                dir = shmem_dir_map(middir);
                        nr_swaps_freed += freed;
                        if (offset)
                                spin_lock(&info->lock);
                        set_page_private(subdir, page_private(subdir) - freed);
                        if (offset)
                                spin_unlock(&info->lock);
                        if (!punch_hole)
                                BUG_ON(page_private(subdir) > offset);
                }
                if (offset)
                        offset = 0;
                else if (subdir && !page_private(subdir)) {
                        dir[diroff] = NULL;
                        nr_pages_to_free++;
                        list_add(&subdir->lru, &pages_to_free);
                }
        }
done1:
        shmem_dir_unmap(dir);
done2:
        if (inode->i_mapping->nrpages && (info->flags & SHMEM_PAGEIN)) {
                /*
                 * Call truncate_inode_pages again: racing shmem_unuse_inode
                 * may have swizzled a page in from swap since vmtruncate or
                 * generic_delete_inode did it, before we lowered next_index.
                 * Also, though shmem_getpage checks i_size before adding to
                 * cache, no recheck after: so fix the narrow window there too.
                 */
                truncate_inode_pages_range(inode->i_mapping, start, end);
        }

        spin_lock(&info->lock);
        info->flags &= ~SHMEM_TRUNCATE;
        info->swapped -= nr_swaps_freed;
        if (nr_pages_to_free)
                shmem_free_blocks(inode, nr_pages_to_free);
        shmem_recalc_inode(inode);
        spin_unlock(&info->lock);

        /*
         * Empty swap vector directory pages to be freed?
         */
        if (!list_empty(&pages_to_free)) {
                pages_to_free.prev->next = NULL;
                shmem_free_pages(pages_to_free.next);
        }
}

static void shmem_truncate(struct inode *inode)
{
        shmem_truncate_range(inode, inode->i_size, (loff_t)-1);
}

static int shmem_notify_change(struct dentry *dentry, struct iattr *attr)
{
        struct inode *inode = dentry->d_inode;
        struct page *page = NULL;
        int error;

        if (attr->ia_valid & ATTR_SIZE) {
                if (attr->ia_size < inode->i_size) {
                        /*
                         * If truncating down to a partial page, then
                         * if that page is already allocated, hold it
                         * in memory until the truncation is over, so
                         * truncate_partial_page cannnot miss it were
                         * it assigned to swap.
                         */
                        if (attr->ia_size & (PAGE_CACHE_SIZE-1)) {
                                (void) shmem_getpage(inode,
                                        attr->ia_size>>PAGE_CACHE_SHIFT,
                                                &page, SGP_READ, NULL);
                        }
                        /*
                         * Reset SHMEM_PAGEIN flag so that shmem_truncate can
                         * detect if any pages might have been added to cache
                         * after truncate_inode_pages.  But we needn't bother
                         * if it's being fully truncated to zero-length: the
                         * nrpages check is efficient enough in that case.
                         */
                        if (attr->ia_size) {
                                struct shmem_inode_info *info = SHMEM_I(inode);
                                spin_lock(&info->lock);
                                info->flags &= ~SHMEM_PAGEIN;
                                spin_unlock(&info->lock);
                        }
                }
        }

        error = inode_change_ok(inode, attr);
        if (!error)
                error = inode_setattr(inode, attr);
        if (page)
                page_cache_release(page);
        return error;
}

static void shmem_delete_inode(struct inode *inode)
{
        struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
        struct shmem_inode_info *info = SHMEM_I(inode);

        if (inode->i_op->truncate == shmem_truncate) {
                truncate_inode_pages(inode->i_mapping, 0);
                shmem_unacct_size(info->flags, inode->i_size);
                inode->i_size = 0;
                shmem_truncate(inode);
                if (!list_empty(&info->swaplist)) {
                        spin_lock(&shmem_swaplist_lock);
                        list_del_init(&info->swaplist);
                        spin_unlock(&shmem_swaplist_lock);
                }
        }
        BUG_ON(inode->i_blocks);
        if (sbinfo->max_inodes) {
                spin_lock(&sbinfo->stat_lock);
                sbinfo->free_inodes++;
                spin_unlock(&sbinfo->stat_lock);
        }
        clear_inode(inode);
}

static inline int shmem_find_swp(swp_entry_t entry, swp_entry_t *dir, swp_entry_t *edir)
{
        swp_entry_t *ptr;

        for (ptr = dir; ptr < edir; ptr++) {
                if (ptr->val == entry.val)
                        return ptr - dir;
        }
        return -1;
}

static int shmem_unuse_inode(struct shmem_inode_info *info, swp_entry_t entry, struct page *page)
{
        struct inode *inode;
        unsigned long idx;
        unsigned long size;
        unsigned long limit;
        unsigned long stage;
        struct page **dir;
        struct page *subdir;
        swp_entry_t *ptr;
        int offset;

        idx = 0;
        ptr = info->i_direct;
        spin_lock(&info->lock);
        limit = info->next_index;
        size = limit;
        if (size > SHMEM_NR_DIRECT)
                size = SHMEM_NR_DIRECT;
        offset = shmem_find_swp(entry, ptr, ptr+size);
        if (offset >= 0) {
                shmem_swp_balance_unmap();
                goto found;
        }
        if (!info->i_indirect)
                goto lost2;

        dir = shmem_dir_map(info->i_indirect);
        stage = SHMEM_NR_DIRECT + ENTRIES_PER_PAGEPAGE/2;

        for (idx = SHMEM_NR_DIRECT; idx < limit; idx += ENTRIES_PER_PAGE, dir++) {
                if (unlikely(idx == stage)) {
                        shmem_dir_unmap(dir-1);
                        dir = shmem_dir_map(info->i_indirect) +
                            ENTRIES_PER_PAGE/2 + idx/ENTRIES_PER_PAGEPAGE;
                        while (!*dir) {
                                dir++;
                                idx += ENTRIES_PER_PAGEPAGE;
                                if (idx >= limit)
                                        goto lost1;
                        }
                        stage = idx + ENTRIES_PER_PAGEPAGE;
                        subdir = *dir;
                        shmem_dir_unmap(dir);
                        dir = shmem_dir_map(subdir);
                }
                subdir = *dir;
                if (subdir && page_private(subdir)) {
                        ptr = shmem_swp_map(subdir);
                        size = limit - idx;
                        if (size > ENTRIES_PER_PAGE)
                                size = ENTRIES_PER_PAGE;
                        offset = shmem_find_swp(entry, ptr, ptr+size);
                        if (offset >= 0) {
                                shmem_dir_unmap(dir);
                                goto found;
                        }
                        shmem_swp_unmap(ptr);
                }
        }
lost1:
        shmem_dir_unmap(dir-1);
lost2:
        spin_unlock(&info->lock);
        return 0;
found:
        idx += offset;
        inode = &info->vfs_inode;
        if (move_from_swap_cache(page, idx, inode->i_mapping) == 0) {
                info->flags |= SHMEM_PAGEIN;
                shmem_swp_set(info, ptr + offset, 0);
        }
        shmem_swp_unmap(ptr);
        spin_unlock(&info->lock);
        /*
         * Decrement swap count even when the entry is left behind:
         * try_to_unuse will skip over mms, then reincrement count.
         */
        swap_free(entry);
        return 1;
}

/*
 * shmem_unuse() search for an eventually swapped out shmem page.
 */
int shmem_unuse(swp_entry_t entry, struct page *page)
{
        struct list_head *p, *next;
        struct shmem_inode_info *info;
        int found = 0;

        spin_lock(&shmem_swaplist_lock);
        list_for_each_safe(p, next, &shmem_swaplist) {
                info = list_entry(p, struct shmem_inode_info, swaplist);
                if (!info->swapped)
                        list_del_init(&info->swaplist);
                else if (shmem_unuse_inode(info, entry, page)) {
                        /* move head to start search for next from here */
                        list_move_tail(&shmem_swaplist, &info->swaplist);
                        found = 1;
                        break;
                }
        }
        spin_unlock(&shmem_swaplist_lock);
        return found;
}

/*
 * Move the page from the page cache to the swap cache.
 */
static int shmem_writepage(struct page *page, struct writeback_control *wbc)
{
        struct shmem_inode_info *info;
        swp_entry_t *entry, swap;
        struct address_space *mapping;
        unsigned long index;
        struct inode *inode;

        BUG_ON(!PageLocked(page));
        BUG_ON(page_mapped(page));

        mapping = page->mapping;
        index = page->index;
        inode = mapping->host;
        info = SHMEM_I(inode);
        if (info->flags & VM_LOCKED)
                goto redirty;
        swap = get_swap_page();
        if (!swap.val)
                goto redirty;

        spin_lock(&info->lock);
        shmem_recalc_inode(inode);
        if (index >= info->next_index) {
                BUG_ON(!(info->flags & SHMEM_TRUNCATE));
                goto unlock;
        }
        entry = shmem_swp_entry(info, index, NULL);
        BUG_ON(!entry);
        BUG_ON(entry->val);

        if (move_to_swap_cache(page, swap) == 0) {
                shmem_swp_set(info, entry, swap.val);
                shmem_swp_unmap(entry);
                spin_unlock(&info->lock);
                if (list_empty(&info->swaplist)) {
                        spin_lock(&shmem_swaplist_lock);
                        /* move instead of add in case we're racing */
                        list_move_tail(&info->swaplist, &shmem_swaplist);
                        spin_unlock(&shmem_swaplist_lock);
                }
                unlock_page(page);
                return 0;
        }

        shmem_swp_unmap(entry);
unlock:
        spin_unlock(&info->lock);
        swap_free(swap);
redirty:
        set_page_dirty(page);
        return AOP_WRITEPAGE_ACTIVATE;  /* Return with the page locked */
}

#ifdef CONFIG_NUMA
static struct page *shmem_swapin_async(struct shared_policy *p,
                                       swp_entry_t entry, unsigned long idx)
{
        struct page *page;
        struct vm_area_struct pvma;

        /* Create a pseudo vma that just contains the policy */
        memset(&pvma, 0, sizeof(struct vm_area_struct));
        pvma.vm_end = PAGE_SIZE;
        pvma.vm_pgoff = idx;
        pvma.vm_policy = mpol_shared_policy_lookup(p, idx);
        page = read_swap_cache_async(entry, &pvma, 0);
        mpol_free(pvma.vm_policy);
        return page;
}

struct page *shmem_swapin(struct shmem_inode_info *info, swp_entry_t entry,
                          unsigned long idx)
{
        struct shared_policy *p = &info->policy;
        int i, num;
        struct page *page;
        unsigned long offset;

        num = valid_swaphandles(entry, &offset);
        for (i = 0; i < num; offset++, i++) {
                page = shmem_swapin_async(p,
                                swp_entry(swp_type(entry), offset), idx);
                if (!page)
                        break;
                page_cache_release(page);
        }
        lru_add_drain();        /* Push any new pages onto the LRU now */
        return shmem_swapin_async(p, entry, idx);
}

static struct page *
shmem_alloc_page(gfp_t gfp, struct shmem_inode_info *info,
                 unsigned long idx)
{
        struct vm_area_struct pvma;
        struct page *page;

        memset(&pvma, 0, sizeof(struct vm_area_struct));
        pvma.vm_policy = mpol_shared_policy_lookup(&info->policy, idx);
        pvma.vm_pgoff = idx;
        pvma.vm_end = PAGE_SIZE;
        page = alloc_page_vma(gfp | __GFP_ZERO, &pvma, 0);
        mpol_free(pvma.vm_policy);
        return page;
}
#else
static inline struct page *
shmem_swapin(struct shmem_inode_info *info,swp_entry_t entry,unsigned long idx)
{
        swapin_readahead(entry, 0, NULL);
        return read_swap_cache_async(entry, NULL, 0);
}

static inline struct page *
shmem_alloc_page(gfp_t gfp,struct shmem_inode_info *info, unsigned long idx)
{
        return alloc_page(gfp | __GFP_ZERO);
}
#endif

/*
 * shmem_getpage - either get the page from swap or allocate a new one
 *
 * If we allocate a new one we do not mark it dirty. That's up to the
 * vm. If we swap it in we mark it dirty since we also free the swap
 * entry since a page cannot live in both the swap and page cache
 */
static int shmem_getpage(struct inode *inode, unsigned long idx,
                        struct page **pagep, enum sgp_type sgp, int *type)
{
        struct address_space *mapping = inode->i_mapping;
        struct shmem_inode_info *info = SHMEM_I(inode);
        struct shmem_sb_info *sbinfo;
        struct page *filepage = *pagep;
        struct page *swappage;
        swp_entry_t *entry;
        swp_entry_t swap;
        int error;

        if (idx >= SHMEM_MAX_INDEX)
                return -EFBIG;
        /*
         * Normally, filepage is NULL on entry, and either found
         * uptodate immediately, or allocated and zeroed, or read
         * in under swappage, which is then assigned to filepage.
         * But shmem_prepare_write passes in a locked filepage,
         * which may be found not uptodate by other callers too,
         * and may need to be copied from the swappage read in.
         */
repeat:
        if (!filepage)
                filepage = find_lock_page(mapping, idx);
        if (filepage && PageUptodate(filepage))
                goto done;
        error = 0;
        if (sgp == SGP_QUICK)
                goto failed;

        spin_lock(&info->lock);
        shmem_recalc_inode(inode);
        entry = shmem_swp_alloc(info, idx, sgp);
        if (IS_ERR(entry)) {
                spin_unlock(&info->lock);
                error = PTR_ERR(entry);
                goto failed;
        }
        swap = *entry;

        if (swap.val) {
                /* Look it up and read it in.. */
                swappage = lookup_swap_cache(swap);
                if (!swappage) {
                        shmem_swp_unmap(entry);
                        spin_unlock(&info->lock);
                        /* here we actually do the io */
                        if (type && *type == VM_FAULT_MINOR) {
                                inc_page_state(pgmajfault);
                                *type = VM_FAULT_MAJOR;
                        }
                        swappage = shmem_swapin(info, swap, idx);
                        if (!swappage) {
                                spin_lock(&info->lock);
                                entry = shmem_swp_alloc(info, idx, sgp);
                                if (IS_ERR(entry))
                                        error = PTR_ERR(entry);
                                else {
                                        if (entry->val == swap.val)
                                                error = -ENOMEM;
                                        shmem_swp_unmap(entry);
                                }
                                spin_unlock(&info->lock);
                                if (error)
                                        goto failed;
                                goto repeat;
                        }
                        wait_on_page_locked(swappage);
                        page_cache_release(swappage);
                        goto repeat;
                }

                /* We have to do this with page locked to prevent races */
                if (TestSetPageLocked(swappage)) {
                        shmem_swp_unmap(entry);
                        spin_unlock(&info->lock);
                        wait_on_page_locked(swappage);
                        page_cache_release(swappage);
                        goto repeat;
                }
                if (PageWriteback(swappage)) {
                        shmem_swp_unmap(entry);
                        spin_unlock(&info->lock);
                        wait_on_page_writeback(swappage);
                        unlock_page(swappage);
                        page_cache_release(swappage);
                        goto repeat;
                }
                if (!PageUptodate(swappage)) {
                        shmem_swp_unmap(entry);
                        spin_unlock(&info->lock);
                        unlock_page(swappage);
                        page_cache_release(swappage);
                        error = -EIO;
                        goto failed;
                }

                if (filepage) {
                        shmem_swp_set(info, entry, 0);
                        shmem_swp_unmap(entry);
                        delete_from_swap_cache(swappage);
                        spin_unlock(&info->lock);
                        copy_highpage(filepage, swappage);
                        unlock_page(swappage);
                        page_cache_release(swappage);
                        flush_dcache_page(filepage);
                        SetPageUptodate(filepage);
                        set_page_dirty(filepage);
                        swap_free(swap);
                } else if (!(error = move_from_swap_cache(
                                swappage, idx, mapping))) {
                        info->flags |= SHMEM_PAGEIN;
                        shmem_swp_set(info, entry, 0);
                        shmem_swp_unmap(entry);
                        spin_unlock(&info->lock);
                        filepage = swappage;
                        swap_free(swap);
                } else {
                        shmem_swp_unmap(entry);
                        spin_unlock(&info->lock);
                        unlock_page(swappage);
                        page_cache_release(swappage);
                        if (error == -ENOMEM) {
                                /* let kswapd refresh zone for GFP_ATOMICs */
                                blk_congestion_wait(WRITE, HZ/50);
                        }
                        goto repeat;
                }
        } else if (sgp == SGP_READ && !filepage) {
                shmem_swp_unmap(entry);
                filepage = find_get_page(mapping, idx);
                if (filepage &&
                    (!PageUptodate(filepage) || TestSetPageLocked(filepage))) {
                        spin_unlock(&info->lock);
                        wait_on_page_locked(filepage);
                        page_cache_release(filepage);
                        filepage = NULL;
                        goto repeat;
                }
                spin_unlock(&info->lock);
        } else {
                shmem_swp_unmap(entry);
                sbinfo = SHMEM_SB(inode->i_sb);
                if (sbinfo->max_blocks) {
                        spin_lock(&sbinfo->stat_lock);
                        if (sbinfo->free_blocks == 0 ||
                            shmem_acct_block(info->flags)) {
                                spin_unlock(&sbinfo->stat_lock);
                                spin_unlock(&info->lock);
                                error = -ENOSPC;
                                goto failed;
                        }
                        sbinfo->free_blocks--;
                        inode->i_blocks += BLOCKS_PER_PAGE;
                        spin_unlock(&sbinfo->stat_lock);
                } else if (shmem_acct_block(info->flags)) {
                        spin_unlock(&info->lock);
                        error = -ENOSPC;
                        goto failed;
                }

                if (!filepage) {
                        spin_unlock(&info->lock);
                        filepage = shmem_alloc_page(mapping_gfp_mask(mapping),
                                                    info,
                                                    idx);
                        if (!filepage) {
                                shmem_unacct_blocks(info->flags, 1);
                                shmem_free_blocks(inode, 1);
                                error = -ENOMEM;
                                goto failed;
                        }

                        spin_lock(&info->lock);
                        entry = shmem_swp_alloc(info, idx, sgp);
                        if (IS_ERR(entry))
                                error = PTR_ERR(entry);
                        else {
                                swap = *entry;
                                shmem_swp_unmap(entry);
                        }
                        if (error || swap.val || 0 != add_to_page_cache_lru(
                                        filepage, mapping, idx, GFP_ATOMIC)) {
                                spin_unlock(&info->lock);
                                page_cache_release(filepage);
                                shmem_unacct_blocks(info->flags, 1);
                                shmem_free_blocks(inode, 1);
                                filepage = NULL;
                                if (error)
                                        goto failed;
                                goto repeat;
                        }
                        info->flags |= SHMEM_PAGEIN;
                }

                info->alloced++;
                spin_unlock(&info->lock);
                flush_dcache_page(filepage);
                SetPageUptodate(filepage);
        }
done:
        if (*pagep != filepage) {
                unlock_page(filepage);
                *pagep = filepage;
        }
        return 0;

failed:
        if (*pagep != filepage) {
                unlock_page(filepage);
                page_cache_release(filepage);
        }
        return error;
}

struct page *shmem_nopage(struct vm_area_struct *vma, unsigned long address, int *type)
{
        struct inode *inode = vma->vm_file->f_dentry->d_inode;
        struct page *page = NULL;
        unsigned long idx;
        int error;

        idx = (address - vma->vm_start) >> PAGE_SHIFT;
        idx += vma->vm_pgoff;
        idx >>= PAGE_CACHE_SHIFT - PAGE_SHIFT;
        if (((loff_t) idx << PAGE_CACHE_SHIFT) >= i_size_read(inode))
                return NOPAGE_SIGBUS;

        error = shmem_getpage(inode, idx, &page, SGP_CACHE, type);
        if (error)
                return (error == -ENOMEM)? NOPAGE_OOM: NOPAGE_SIGBUS;

        mark_page_accessed(page);
        return page;
}

static int shmem_populate(struct vm_area_struct *vma,
        unsigned long addr, unsigned long len,
        pgprot_t prot, unsigned long pgoff, int nonblock)
{
        struct inode *inode = vma->vm_file->f_dentry->d_inode;
        struct mm_struct *mm = vma->vm_mm;
        enum sgp_type sgp = nonblock? SGP_QUICK: SGP_CACHE;
        unsigned long size;

        size = (i_size_read(inode) + PAGE_SIZE - 1) >> PAGE_SHIFT;
        if (pgoff >= size || pgoff + (len >> PAGE_SHIFT) > size)
                return -EINVAL;

        while ((long) len > 0) {
                struct page *page = NULL;
                int err;
                /*
                 * Will need changing if PAGE_CACHE_SIZE != PAGE_SIZE
                 */
                err = shmem_getpage(inode, pgoff, &page, sgp, NULL);
                if (err)
                        return err;
                /* Page may still be null, but only if nonblock was set. */
                if (page) {
                        mark_page_accessed(page);
                        err = install_page(mm, vma, addr, page, prot);
                        if (err) {
                                page_cache_release(page);
                                return err;
                        }
                } else if (vma->vm_flags & VM_NONLINEAR) {
                        /* No page was found just because we can't read it in
                         * now (being here implies nonblock != 0), but the page
                         * may exist, so set the PTE to fault it in later. */
                        err = install_file_pte(mm, vma, addr, pgoff, prot);
                        if (err)
                                return err;
                }

                len -= PAGE_SIZE;
                addr += PAGE_SIZE;
                pgoff++;
        }
        return 0;
}

#ifdef CONFIG_NUMA
int shmem_set_policy(struct vm_area_struct *vma, struct mempolicy *new)
{
        struct inode *i = vma->vm_file->f_dentry->d_inode;
        return mpol_set_shared_policy(&SHMEM_I(i)->policy, vma, new);
}

struct mempolicy *
shmem_get_policy(struct vm_area_struct *vma, unsigned long addr)
{
        struct inode *i = vma->vm_file->f_dentry->d_inode;
        unsigned long idx;

        idx = ((addr - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
        return mpol_shared_policy_lookup(&SHMEM_I(i)->policy, idx);
}
#endif

int shmem_lock(struct file *file, int lock, struct user_struct *user)
{
        struct inode *inode = file->f_dentry->d_inode;
        struct shmem_inode_info *info = SHMEM_I(inode);
        int retval = -ENOMEM;

        spin_lock(&info->lock);
        if (lock && !(info->flags & VM_LOCKED)) {
                if (!user_shm_lock(inode->i_size, user))
                        goto out_nomem;
                info->flags |= VM_LOCKED;
        }
        if (!lock && (info->flags & VM_LOCKED) && user) {
                user_shm_unlock(inode->i_size, user);
                info->flags &= ~VM_LOCKED;
        }
        retval = 0;
out_nomem:
        spin_unlock(&info->lock);
        return retval;
}

int shmem_mmap(struct file *file, struct vm_area_struct *vma)
{
        file_accessed(file);
        vma->vm_ops = &shmem_vm_ops;
        return 0;
}

static struct inode *
shmem_get_inode(struct super_block *sb, int mode, dev_t dev)
{
        struct inode *inode;
        struct shmem_inode_info *info;
        struct shmem_sb_info *sbinfo = SHMEM_SB(sb);

        if (sbinfo->max_inodes) {
                spin_lock(&sbinfo->stat_lock);
                if (!sbinfo->free_inodes) {
                        spin_unlock(&sbinfo->stat_lock);
                        return NULL;
                }
                sbinfo->free_inodes--;
                spin_unlock(&sbinfo->stat_lock);
        }

        inode = new_inode(sb);
        if (inode) {
                inode->i_mode = mode;
                inode->i_uid = current->fsuid;
                inode->i_gid = current->fsgid;
                inode->i_blksize = PAGE_CACHE_SIZE;
                inode->i_blocks = 0;
                inode->i_mapping->a_ops = &shmem_aops;
                inode->i_mapping->backing_dev_info = &shmem_backing_dev_info;
                inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
                info = SHMEM_I(inode);
                memset(info, 0, (char *)inode - (char *)info);
                spin_lock_init(&info->lock);
                INIT_LIST_HEAD(&info->swaplist);

                switch (mode & S_IFMT) {
                default:
                        init_special_inode(inode, mode, dev);
                        break;
                case S_IFREG:
                        inode->i_op = &shmem_inode_operations;
                        inode->i_fop = &shmem_file_operations;
                        mpol_shared_policy_init(&info->policy, sbinfo->policy,
                                                        &sbinfo->policy_nodes);
                        break;
                case S_IFDIR:
                        inode->i_nlink++;
                        /* Some things misbehave if size == 0 on a directory */
                        inode->i_size = 2 * BOGO_DIRENT_SIZE;
                        inode->i_op = &shmem_dir_inode_operations;
                        inode->i_fop = &simple_dir_operations;
                        break;
                case S_IFLNK:
                        /*
                         * Must not load anything in the rbtree,
                         * mpol_free_shared_policy will not be called.
                         */
                        mpol_shared_policy_init(&info->policy, MPOL_DEFAULT,
                                                NULL);
                        break;
                }
        } else if (sbinfo->max_inodes) {
                spin_lock(&sbinfo->stat_lock);
                sbinfo->free_inodes++;
                spin_unlock(&sbinfo->stat_lock);
        }
        return inode;
}

#ifdef CONFIG_TMPFS
static struct inode_operations shmem_symlink_inode_operations;
static struct inode_operations shmem_symlink_inline_operations;

/*
 * Normally tmpfs makes no use of shmem_prepare_write, but it
 * lets a tmpfs file be used read-write below the loop driver.
 */
static int
shmem_prepare_write(struct file *file, struct page *page, unsigned offset, unsigned to)
{
        struct inode *inode = page->mapping->host;
        return shmem_getpage(inode, page->index, &page, SGP_WRITE, NULL);
}

static ssize_t
shmem_file_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos)
{
        struct inode    *inode = file->f_dentry->d_inode;
        loff_t          pos;
        unsigned long   written;
        ssize_t         err;

        if ((ssize_t) count < 0)
                return -EINVAL;

        if (!access_ok(VERIFY_READ, buf, count))
                return -EFAULT;

        mutex_lock(&inode->i_mutex);

        pos = *ppos;
        written = 0;

        err = generic_write_checks(file, &pos, &count, 0);
        if (err || !count)
                goto out;

        err = remove_suid(file->f_dentry);
        if (err)
                goto out;

        inode->i_ctime = inode->i_mtime = CURRENT_TIME;

        do {
                struct page *page = NULL;
                unsigned long bytes, index, offset;
                char *kaddr;
                int left;

                offset = (pos & (PAGE_CACHE_SIZE -1)); /* Within page */
                index = pos >> PAGE_CACHE_SHIFT;
                bytes = PAGE_CACHE_SIZE - offset;
                if (bytes > count)
                        bytes = count;

                /*
                 * We don't hold page lock across copy from user -
                 * what would it guard against? - so no deadlock here.
                 * But it still may be a good idea to prefault below.
                 */

                err = shmem_getpage(inode, index, &page, SGP_WRITE, NULL);
                if (err)
                        break;

                left = bytes;
                if (PageHighMem(page)) {
                        volatile unsigned char dummy;
                        __get_user(dummy, buf);
                        __get_user(dummy, buf + bytes - 1);

                        kaddr = kmap_atomic(page, KM_USER0);
                        left = __copy_from_user_inatomic(kaddr + offset,
                                                        buf, bytes);
                        kunmap_atomic(kaddr, KM_USER0);
                }
                if (left) {
                        kaddr = kmap(page);
                        left = __copy_from_user(kaddr + offset, buf, bytes);
                        kunmap(page);
                }

                written += bytes;
                count -= bytes;
                pos += bytes;
                buf += bytes;
                if (pos > inode->i_size)
                        i_size_write(inode, pos);

                flush_dcache_page(page);
                set_page_dirty(page);
                mark_page_accessed(page);
                page_cache_release(page);

                if (left) {
                        pos -= left;
                        written -= left;
                        err = -EFAULT;
                        break;
                }

                /*
                 * Our dirty pages are not counted in nr_dirty,
                 * and we do not attempt to balance dirty pages.
                 */

                cond_resched();
        } while (count);

        *ppos = pos;
        if (written)
                err = written;
out:
        mutex_unlock(&inode->i_mutex);
        return err;
}

static void do_shmem_file_read(struct file *filp, loff_t *ppos, read_descriptor_t *desc, read_actor_t actor)
{
        struct inode *inode = filp->f_dentry->d_inode;
        struct address_space *mapping = inode->i_mapping;
        unsigned long index, offset;

        index = *ppos >> PAGE_CACHE_SHIFT;
        offset = *ppos & ~PAGE_CACHE_MASK;

        for (;;) {
                struct page *page = NULL;
                unsigned long end_index, nr, ret;
                loff_t i_size = i_size_read(inode);

                end_index = i_size >> PAGE_CACHE_SHIFT;
                if (index > end_index)
                        break;
                if (index == end_index) {
                        nr = i_size & ~PAGE_CACHE_MASK;
                        if (nr <= offset)
                                break;
                }

                desc->error = shmem_getpage(inode, index, &page, SGP_READ, NULL);
                if (desc->error) {
                        if (desc->error == -EINVAL)
                                desc->error = 0;
                        break;
                }

                /*
                 * We must evaluate after, since reads (unlike writes)
                 * are called without i_mutex protection against truncate
                 */
                nr = PAGE_CACHE_SIZE;
                i_size = i_size_read(inode);
                end_index = i_size >> PAGE_CACHE_SHIFT;
                if (index == end_index) {
                        nr = i_size & ~PAGE_CACHE_MASK;
                        if (nr <= offset) {
                                if (page)
                                        page_cache_release(page);
                                break;
                        }
                }
                nr -= offset;

                if (page) {
                        /*
                         * If users can be writing to this page using arbitrary
                         * virtual addresses, take care about potential aliasing
                         * before reading the page on the kernel side.
                         */
                        if (mapping_writably_mapped(mapping))
                                flush_dcache_page(page);
                        /*
                         * Mark the page accessed if we read the beginning.
                         */
                        if (!offset)
                                mark_page_accessed(page);
                } else {
                        page = ZERO_PAGE(0);
                        page_cache_get(page);
                }

                /*
                 * Ok, we have the page, and it's up-to-date, so
                 * now we can copy it to user space...
                 *
                 * The actor routine returns how many bytes were actually used..
                 * NOTE! This may not be the same as how much of a user buffer
                 * we filled up (we may be padding etc), so we can only update
                 * "pos" here (the actor routine has to update the user buffer
                 * pointers and the remaining count).
                 */
                ret = actor(desc, page, offset, nr);
                offset += ret;
                index += offset >> PAGE_CACHE_SHIFT;
                offset &= ~PAGE_CACHE_MASK;

                page_cache_release(page);
                if (ret != nr || !desc->count)
                        break;

                cond_resched();
        }

        *ppos = ((loff_t) index << PAGE_CACHE_SHIFT) + offset;
        file_accessed(filp);
}

static ssize_t shmem_file_read(struct file *filp, char __user *buf, size_t count, loff_t *ppos)
{
        read_descriptor_t desc;

        if ((ssize_t) count < 0)
                return -EINVAL;
        if (!access_ok(VERIFY_WRITE, buf, count))
                return -EFAULT;
        if (!count)
                return 0;

        desc.written = 0;
        desc.count = count;
        desc.arg.buf = buf;
        desc.error = 0;

        do_shmem_file_read(filp, ppos, &desc, file_read_actor);
        if (desc.written)
                return desc.written;
        return desc.error;
}

static ssize_t shmem_file_sendfile(struct file *in_file, loff_t *ppos,
                         size_t count, read_actor_t actor, void *target)
{
        read_descriptor_t desc;

        if (!count)
                return 0;

        desc.written = 0;
        desc.count = count;
        desc.arg.data = target;
        desc.error = 0;

        do_shmem_file_read(in_file, ppos, &desc, actor);
        if (desc.written)
                return desc.written;
        return desc.error;
}

static int shmem_statfs(struct super_block *sb, struct kstatfs *buf)
{
        struct shmem_sb_info *sbinfo = SHMEM_SB(sb);

        buf->f_type = TMPFS_MAGIC;
        buf->f_bsize = PAGE_CACHE_SIZE;
        buf->f_namelen = NAME_MAX;
        spin_lock(&sbinfo->stat_lock);
        if (sbinfo->max_blocks) {
                buf->f_blocks = sbinfo->max_blocks;
                buf->f_bavail = buf->f_bfree = sbinfo->free_blocks;
        }
        if (sbinfo->max_inodes) {
                buf->f_files = sbinfo->max_inodes;
                buf->f_ffree = sbinfo->free_inodes;
        }
        /* else leave those fields 0 like simple_statfs */
        spin_unlock(&sbinfo->stat_lock);
        return 0;
}

/*
 * File creation. Allocate an inode, and we're done..
 */
static int
shmem_mknod(struct inode *dir, struct dentry *dentry, int mode, dev_t dev)
{
        struct inode *inode = shmem_get_inode(dir->i_sb, mode, dev);
        int error = -ENOSPC;

        if (inode) {
                error = security_inode_init_security(inode, dir, NULL, NULL,
                                                     NULL);
                if (error) {
                        if (error != -EOPNOTSUPP) {
                                iput(inode);
                                return error;
                        }
                        error = 0;
                }
                if (dir->i_mode & S_ISGID) {
                        inode->i_gid = dir->i_gid;
                        if (S_ISDIR(mode))
                                inode->i_mode |= S_ISGID;
                }
                dir->i_size += BOGO_DIRENT_SIZE;
                dir->i_ctime = dir->i_mtime = CURRENT_TIME;
                d_instantiate(dentry, inode);
                dget(dentry); /* Extra count - pin the dentry in core */
        }
        return error;
}

static int shmem_mkdir(struct inode *dir, struct dentry *dentry, int mode)
{
        int error;

        if ((error = shmem_mknod(dir, dentry, mode | S_IFDIR, 0)))
                return error;
        dir->i_nlink++;
        return 0;
}

static int shmem_create(struct inode *dir, struct dentry *dentry, int mode,
                struct nameidata *nd)
{
        return shmem_mknod(dir, dentry, mode | S_IFREG, 0);
}

/*
 * Link a file..
 */
static int shmem_link(struct dentry *old_dentry, struct inode *dir, struct dentry *dentry)
{
        struct inode *inode = old_dentry->d_inode;
        struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);

        /*
         * No ordinary (disk based) filesystem counts links as inodes;
         * but each new link needs a new dentry, pinning lowmem, and
         * tmpfs dentries cannot be pruned until they are unlinked.
         */
        if (sbinfo->max_inodes) {
                spin_lock(&sbinfo->stat_lock);
                if (!sbinfo->free_inodes) {
                        spin_unlock(&sbinfo->stat_lock);
                        return -ENOSPC;
                }
                sbinfo->free_inodes--;
                spin_unlock(&sbinfo->stat_lock);
        }

        dir->i_size += BOGO_DIRENT_SIZE;
        inode->i_ctime = dir->i_ctime = dir->i_mtime = CURRENT_TIME;
        inode->i_nlink++;
        atomic_inc(&inode->i_count);    /* New dentry reference */
        dget(dentry);           /* Extra pinning count for the created dentry */
        d_instantiate(dentry, inode);
        return 0;
}

static int shmem_unlink(struct inode *dir, struct dentry *dentry)
{
        struct inode *inode = dentry->d_inode;

        if (inode->i_nlink > 1 && !S_ISDIR(inode->i_mode)) {
                struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
                if (sbinfo->max_inodes) {
                        spin_lock(&sbinfo->stat_lock);
                        sbinfo->free_inodes++;
                        spin_unlock(&sbinfo->stat_lock);
                }
        }

        dir->i_size -= BOGO_DIRENT_SIZE;
        inode->i_ctime = dir->i_ctime = dir->i_mtime = CURRENT_TIME;
        inode->i_nlink--;
        dput(dentry);   /* Undo the count from "create" - this does all the work */
        return 0;
}

static int shmem_rmdir(struct inode *dir, struct dentry *dentry)
{
        if (!simple_empty(dentry))
                return -ENOTEMPTY;

        dir->i_nlink--;
        return shmem_unlink(dir, dentry);
}

/*
 * The VFS layer already does all the dentry stuff for rename,
 * we just have to decrement the usage count for the target if
 * it exists so that the VFS layer correctly free's it when it
 * gets overwritten.
 */
static int shmem_rename(struct inode *old_dir, struct dentry *old_dentry, struct inode *new_dir, struct dentry *new_dentry)
{
        struct inode *inode = old_dentry->d_inode;
        int they_are_dirs = S_ISDIR(inode->i_mode);

        if (!simple_empty(new_dentry))
                return -ENOTEMPTY;

        if (new_dentry->d_inode) {
                (void) shmem_unlink(new_dir, new_dentry);
                if (they_are_dirs)
                        old_dir->i_nlink--;
        } else if (they_are_dirs) {
                old_dir->i_nlink--;
                new_dir->i_nlink++;
        }

        old_dir->i_size -= BOGO_DIRENT_SIZE;
        new_dir->i_size += BOGO_DIRENT_SIZE;
        old_dir->i_ctime = old_dir->i_mtime =
        new_dir->i_ctime = new_dir->i_mtime =
        inode->i_ctime = CURRENT_TIME;
        return 0;
}

static int shmem_symlink(struct inode *dir, struct dentry *dentry, const char *symname)
{
        int error;
        int len;
        struct inode *inode;
        struct page *page = NULL;
        char *kaddr;
        struct shmem_inode_info *info;

        len = strlen(symname) + 1;
        if (len > PAGE_CACHE_SIZE)
                return -ENAMETOOLONG;

        inode = shmem_get_inode(dir->i_sb, S_IFLNK|S_IRWXUGO, 0);
        if (!inode)
                return -ENOSPC;

        error = security_inode_init_security(inode, dir, NULL, NULL,
                                             NULL);
        if (error) {
                if (error != -EOPNOTSUPP) {
                        iput(inode);
                        return error;
                }
                error = 0;
        }

        info = SHMEM_I(inode);
        inode->i_size = len-1;
        if (len <= (char *)inode - (char *)info) {
                /* do it inline */
                memcpy(info, symname, len);
                inode->i_op = &shmem_symlink_inline_operations;
        } else {
                error = shmem_getpage(inode, 0, &page, SGP_WRITE, NULL);
                if (error) {
                        iput(inode);
                        return error;
                }
                inode->i_op = &shmem_symlink_inode_operations;
                kaddr = kmap_atomic(page, KM_USER0);
                memcpy(kaddr, symname, len);
                kunmap_atomic(kaddr, KM_USER0);
                set_page_dirty(page);
                page_cache_release(page);
        }
        if (dir->i_mode & S_ISGID)
                inode->i_gid = dir->i_gid;
        dir->i_size += BOGO_DIRENT_SIZE;
        dir->i_ctime = dir->i_mtime = CURRENT_TIME;
        d_instantiate(dentry, inode);
        dget(dentry);
        return 0;
}

static void *shmem_follow_link_inline(struct dentry *dentry, struct nameidata *nd)
{
        nd_set_link(nd, (char *)SHMEM_I(dentry->d_inode));
        return NULL;
}

static void *shmem_follow_link(struct dentry *dentry, struct nameidata *nd)
{
        struct page *page = NULL;
        int res = shmem_getpage(dentry->d_inode, 0, &page, SGP_READ, NULL);
        nd_set_link(nd, res ? ERR_PTR(res) : kmap(page));
        return page;
}

static void shmem_put_link(struct dentry *dentry, struct nameidata *nd, void *cookie)
{
        if (!IS_ERR(nd_get_link(nd))) {
                struct page *page = cookie;
                kunmap(page);
                mark_page_accessed(page);
                page_cache_release(page);
        }
}

static struct inode_operations shmem_symlink_inline_operations = {
        .readlink       = generic_readlink,
        .follow_link    = shmem_follow_link_inline,
};

static struct inode_operations shmem_symlink_inode_operations = {
        .truncate       = shmem_truncate,
        .readlink       = generic_readlink,
        .follow_link    = shmem_follow_link,
        .put_link       = shmem_put_link,
};

static int shmem_parse_options(char *options, int *mode, uid_t *uid,
        gid_t *gid, unsigned long *blocks, unsigned long *inodes,
        int *policy, nodemask_t *policy_nodes)
{
        char *this_char, *value, *rest;

        while ((this_char = strsep(&options, ",")) != NULL) {
                if (!*this_char)
                        continue;
                if ((value = strchr(this_char,'=')) != NULL) {
                        *value++ = 0;
                } else {
                        printk(KERN_ERR
                            "tmpfs: No value for mount option '%s'\n",
                            this_char);
                        return 1;
                }

                if (!strcmp(this_char,"size")) {
                        unsigned long long size;
                        size = memparse(value,&rest);
                        if (*rest == '%') {
                                size <<= PAGE_SHIFT;
                                size *= totalram_pages;
                                do_div(size, 100);
                                rest++;
                        }
                        if (*rest)
                                goto bad_val;
                        *blocks = size >> PAGE_CACHE_SHIFT;
                } else if (!strcmp(this_char,"nr_blocks")) {
                        *blocks = memparse(value,&rest);
                        if (*rest)
                                goto bad_val;
                } else if (!strcmp(this_char,"nr_inodes")) {
                        *inodes = memparse(value,&rest);
                        if (*rest)
                                goto bad_val;
                } else if (!strcmp(this_char,"mode")) {
                        if (!mode)
                                continue;
                        *mode = simple_strtoul(value,&rest,8);
                        if (*rest)
                                goto bad_val;
                } else if (!strcmp(this_char,"uid")) {
                        if (!uid)
                                continue;
                        *uid = simple_strtoul(value,&rest,0);
                        if (*rest)
                                goto bad_val;
                } else if (!strcmp(this_char,"gid")) {
                        if (!gid)
                                continue;
                        *gid = simple_strtoul(value,&rest,0);
                        if (*rest)
                                goto bad_val;
                } else if (!strcmp(this_char,"mpol")) {
                        if (!strcmp(value,"default"))
                                *policy = MPOL_DEFAULT;
                        else if (!strcmp(value,"preferred"))
                                *policy = MPOL_PREFERRED;
                        else if (!strcmp(value,"bind"))
                                *policy = MPOL_BIND;
                        else if (!strcmp(value,"interleave"))
                                *policy = MPOL_INTERLEAVE;
                        else
                                goto bad_val;
                } else if (!strcmp(this_char,"mpol_nodelist")) {
                        nodelist_parse(value, *policy_nodes);
                } else {
                        printk(KERN_ERR "tmpfs: Bad mount option %s\n",
                               this_char);
                        return 1;
                }
        }
        return 0;

bad_val:
        printk(KERN_ERR "tmpfs: Bad value '%s' for mount option '%s'\n",
               value, this_char);
        return 1;

}

static int shmem_remount_fs(struct super_block *sb, int *flags, char *data)
{
        struct shmem_sb_info *sbinfo = SHMEM_SB(sb);
        unsigned long max_blocks = sbinfo->max_blocks;
        unsigned long max_inodes = sbinfo->max_inodes;
        int policy = sbinfo->policy;
        nodemask_t policy_nodes = sbinfo->policy_nodes;
        unsigned long blocks;
        unsigned long inodes;
        int error = -EINVAL;

        if (shmem_parse_options(data, NULL, NULL, NULL, &max_blocks,
                                &max_inodes, &policy, &policy_nodes))
                return error;

        spin_lock(&sbinfo->stat_lock);
        blocks = sbinfo->max_blocks - sbinfo->free_blocks;
        inodes = sbinfo->max_inodes - sbinfo->free_inodes;
        if (max_blocks < blocks)
                goto out;
        if (max_inodes < inodes)
                goto out;
        /*
         * Those tests also disallow limited->unlimited while any are in
         * use, so i_blocks will always be zero when max_blocks is zero;
         * but we must separately disallow unlimited->limited, because
         * in that case we have no record of how much is already in use.
         */
        if (max_blocks && !sbinfo->max_blocks)
                goto out;
        if (max_inodes && !sbinfo->max_inodes)
                goto out;

        error = 0;
        sbinfo->max_blocks  = max_blocks;
        sbinfo->free_blocks = max_blocks - blocks;
        sbinfo->max_inodes  = max_inodes;
        sbinfo->free_inodes = max_inodes - inodes;
        sbinfo->policy = policy;
        sbinfo->policy_nodes = policy_nodes;
out:
        spin_unlock(&sbinfo->stat_lock);
        return error;
}
#endif

static void shmem_put_super(struct super_block *sb)
{
        kfree(sb->s_fs_info);
        sb->s_fs_info = NULL;
}

static int shmem_fill_super(struct super_block *sb,
                            void *data, int silent)
{
        struct inode *inode;
        struct dentry *root;
        int mode   = S_IRWXUGO | S_ISVTX;
        uid_t uid = current->fsuid;
        gid_t gid = current->fsgid;
        int err = -ENOMEM;
        struct shmem_sb_info *sbinfo;
        unsigned long blocks = 0;
        unsigned long inodes = 0;
        int policy = MPOL_DEFAULT;
        nodemask_t policy_nodes = node_online_map;

#ifdef CONFIG_TMPFS
        /*
         * Per default we only allow half of the physical ram per
         * tmpfs instance, limiting inodes to one per page of lowmem;
         * but the internal instance is left unlimited.
         */
        if (!(sb->s_flags & MS_NOUSER)) {
                blocks = totalram_pages / 2;
                inodes = totalram_pages - totalhigh_pages;
                if (inodes > blocks)
                        inodes = blocks;
                if (shmem_parse_options(data, &mode, &uid, &gid, &blocks,
                                        &inodes, &policy, &policy_nodes))
                        return -EINVAL;
        }
#else
        sb->s_flags |= MS_NOUSER;
#endif

        /* Round up to L1_CACHE_BYTES to resist false sharing */
        sbinfo = kmalloc(max((int)sizeof(struct shmem_sb_info),
                                L1_CACHE_BYTES), GFP_KERNEL);
        if (!sbinfo)
                return -ENOMEM;

        spin_lock_init(&sbinfo->stat_lock);
        sbinfo->max_blocks = blocks;
        sbinfo->free_blocks = blocks;
        sbinfo->max_inodes = inodes;
        sbinfo->free_inodes = inodes;
        sbinfo->policy = policy;
        sbinfo->policy_nodes = policy_nodes;

        sb->s_fs_info = sbinfo;
        sb->s_maxbytes = SHMEM_MAX_BYTES;
        sb->s_blocksize = PAGE_CACHE_SIZE;
        sb->s_blocksize_bits = PAGE_CACHE_SHIFT;
        sb->s_magic = TMPFS_MAGIC;
        sb->s_op = &shmem_ops;

        inode = shmem_get_inode(sb, S_IFDIR | mode, 0);
        if (!inode)
                goto failed;
        inode->i_uid = uid;
        inode->i_gid = gid;
        root = d_alloc_root(inode);
        if (!root)
                goto failed_iput;
        sb->s_root = root;
        return 0;

failed_iput:
        iput(inode);
failed:
        shmem_put_super(sb);
        return err;
}

static kmem_cache_t *shmem_inode_cachep;

static struct inode *shmem_alloc_inode(struct super_block *sb)
{
        struct shmem_inode_info *p;
        p = (struct shmem_inode_info *)kmem_cache_alloc(shmem_inode_cachep, SLAB_KERNEL);
        if (!p)
                return NULL;
        return &p->vfs_inode;
}

static void shmem_destroy_inode(struct inode *inode)
{
        if ((inode->i_mode & S_IFMT) == S_IFREG) {
                /* only struct inode is valid if it's an inline symlink */
                mpol_free_shared_policy(&SHMEM_I(inode)->policy);
        }
        kmem_cache_free(shmem_inode_cachep, SHMEM_I(inode));
}

static void init_once(void *foo, kmem_cache_t *cachep, unsigned long flags)
{
        struct shmem_inode_info *p = (struct shmem_inode_info *) foo;

        if ((flags & (SLAB_CTOR_VERIFY|SLAB_CTOR_CONSTRUCTOR)) ==
            SLAB_CTOR_CONSTRUCTOR) {
                inode_init_once(&p->vfs_inode);
        }
}

static int init_inodecache(void)
{
        shmem_inode_cachep = kmem_cache_create("shmem_inode_cache",
                                sizeof(struct shmem_inode_info),
                                0, 0, init_once, NULL);
        if (shmem_inode_cachep == NULL)
                return -ENOMEM;
        return 0;
}

static void destroy_inodecache(void)
{
        if (kmem_cache_destroy(shmem_inode_cachep))
                printk(KERN_INFO "shmem_inode_cache: not all structures were freed\n");
}

static struct address_space_operations shmem_aops = {
        .writepage      = shmem_writepage,
        .set_page_dirty = __set_page_dirty_nobuffers,
#ifdef CONFIG_TMPFS
        .prepare_write  = shmem_prepare_write,
        .commit_write   = simple_commit_write,
#endif
};

static struct file_operations shmem_file_operations = {
        .mmap           = shmem_mmap,
#ifdef CONFIG_TMPFS
        .llseek         = generic_file_llseek,
        .read           = shmem_file_read,
        .write          = shmem_file_write,
        .fsync          = simple_sync_file,
        .sendfile       = shmem_file_sendfile,
#endif
};

static struct inode_operations shmem_inode_operations = {
        .truncate       = shmem_truncate,
        .setattr        = shmem_notify_change,
        .truncate_range = shmem_truncate_range,
};

static struct inode_operations shmem_dir_inode_operations = {
#ifdef CONFIG_TMPFS
        .create         = shmem_create,
        .lookup         = simple_lookup,
        .link           = shmem_link,
        .unlink         = shmem_unlink,
        .symlink        = shmem_symlink,
        .mkdir          = shmem_mkdir,
        .rmdir          = shmem_rmdir,
        .mknod          = shmem_mknod,
        .rename         = shmem_rename,
#endif
};

static struct super_operations shmem_ops = {
        .alloc_inode    = shmem_alloc_inode,
        .destroy_inode  = shmem_destroy_inode,
#ifdef CONFIG_TMPFS
        .statfs         = shmem_statfs,
        .remount_fs     = shmem_remount_fs,
#endif
        .delete_inode   = shmem_delete_inode,
        .drop_inode     = generic_delete_inode,
        .put_super      = shmem_put_super,
};

static struct vm_operations_struct shmem_vm_ops = {
        .nopage         = shmem_nopage,
        .populate       = shmem_populate,
#ifdef CONFIG_NUMA
        .set_policy     = shmem_set_policy,
        .get_policy     = shmem_get_policy,
#endif
};


static struct super_block *shmem_get_sb(struct file_system_type *fs_type,
        int flags, const char *dev_name, void *data)
{
        return get_sb_nodev(fs_type, flags, data, shmem_fill_super);
}

static struct file_system_type tmpfs_fs_type = {
        .owner          = THIS_MODULE,
        .name           = "tmpfs",
        .get_sb         = shmem_get_sb,
        .kill_sb        = kill_litter_super,
};
static struct vfsmount *shm_mnt;

static int __init init_tmpfs(void)
{
        int error;

        error = init_inodecache();
        if (error)
                goto out3;

        error = register_filesystem(&tmpfs_fs_type);
        if (error) {
                printk(KERN_ERR "Could not register tmpfs\n");
                goto out2;
        }
#ifdef CONFIG_TMPFS
        devfs_mk_dir("shm");
#endif
        shm_mnt = do_kern_mount(tmpfs_fs_type.name, MS_NOUSER,
                                tmpfs_fs_type.name, NULL);
        if (IS_ERR(shm_mnt)) {
                error = PTR_ERR(shm_mnt);
                printk(KERN_ERR "Could not kern_mount tmpfs\n");
                goto out1;
        }
        return 0;

out1:
        unregister_filesystem(&tmpfs_fs_type);
out2:
        destroy_inodecache();
out3:
        shm_mnt = ERR_PTR(error);
        return error;
}
module_init(init_tmpfs)

/*
 * shmem_file_setup - get an unlinked file living in tmpfs
 *
 * @name: name for dentry (to be seen in /proc/<pid>/maps
 * @size: size to be set for the file
 *
 */
struct file *shmem_file_setup(char *name, loff_t size, unsigned long flags)
{
        int error;
        struct file *file;
        struct inode *inode;
        struct dentry *dentry, *root;
        struct qstr this;

        if (IS_ERR(shm_mnt))
                return (void *)shm_mnt;

        if (size < 0 || size > SHMEM_MAX_BYTES)
                return ERR_PTR(-EINVAL);

        if (shmem_acct_size(flags, size))
                return ERR_PTR(-ENOMEM);

        error = -ENOMEM;
        this.name = name;
        this.len = strlen(name);
        this.hash = 0; /* will go */
        root = shm_mnt->mnt_root;
        dentry = d_alloc(root, &this);
        if (!dentry)
                goto put_memory;

        error = -ENFILE;
        file = get_empty_filp();
        if (!file)
                goto put_dentry;

        error = -ENOSPC;
        inode = shmem_get_inode(root->d_sb, S_IFREG | S_IRWXUGO, 0);
        if (!inode)
                goto close_file;

        SHMEM_I(inode)->flags = flags & VM_ACCOUNT;
        d_instantiate(dentry, inode);
        inode->i_size = size;
        inode->i_nlink = 0;     /* It is unlinked */
        file->f_vfsmnt = mntget(shm_mnt);
        file->f_dentry = dentry;
        file->f_mapping = inode->i_mapping;
        file->f_op = &shmem_file_operations;
        file->f_mode = FMODE_WRITE | FMODE_READ;
        return file;

close_file:
        put_filp(file);
put_dentry:
        dput(dentry);
put_memory:
        shmem_unacct_size(flags, size);
        return ERR_PTR(error);
}

/*
 * shmem_zero_setup - setup a shared anonymous mapping
 *
 * @vma: the vma to be mmapped is prepared by do_mmap_pgoff
 */
int shmem_zero_setup(struct vm_area_struct *vma)
{
        struct file *file;
        loff_t size = vma->vm_end - vma->vm_start;

        file = shmem_file_setup("dev/zero", size, vma->vm_flags);
        if (IS_ERR(file))
                return PTR_ERR(file);

        if (vma->vm_file)
                fput(vma->vm_file);
        vma->vm_file = file;
        vma->vm_ops = &shmem_vm_ops;
        return 0;
}