percpu: make 4k first chunk allocator map memory

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

/*
 * linux/mm/percpu.c - percpu memory allocator
 *
 * Copyright (C) 2009           SUSE Linux Products GmbH
 * Copyright (C) 2009           Tejun Heo <tj@kernel.org>
 *
 * This file is released under the GPLv2.
 *
 * This is percpu allocator which can handle both static and dynamic
 * areas.  Percpu areas are allocated in chunks in vmalloc area.  Each
 * chunk is consisted of num_possible_cpus() units and the first chunk
 * is used for static percpu variables in the kernel image (special
 * boot time alloc/init handling necessary as these areas need to be
 * brought up before allocation services are running).  Unit grows as
 * necessary and all units grow or shrink in unison.  When a chunk is
 * filled up, another chunk is allocated.  ie. in vmalloc area
 *
 *  c0                           c1                         c2
 *  -------------------          -------------------        ------------
 * | u0 | u1 | u2 | u3 |        | u0 | u1 | u2 | u3 |      | u0 | u1 | u
 *  -------------------  ......  -------------------  ....  ------------
 *
 * Allocation is done in offset-size areas of single unit space.  Ie,
 * an area of 512 bytes at 6k in c1 occupies 512 bytes at 6k of c1:u0,
 * c1:u1, c1:u2 and c1:u3.  Percpu access can be done by configuring
 * percpu base registers pcpu_unit_size apart.
 *
 * There are usually many small percpu allocations many of them as
 * small as 4 bytes.  The allocator organizes chunks into lists
 * according to free size and tries to allocate from the fullest one.
 * Each chunk keeps the maximum contiguous area size hint which is
 * guaranteed to be eqaul to or larger than the maximum contiguous
 * area in the chunk.  This helps the allocator not to iterate the
 * chunk maps unnecessarily.
 *
 * Allocation state in each chunk is kept using an array of integers
 * on chunk->map.  A positive value in the map represents a free
 * region and negative allocated.  Allocation inside a chunk is done
 * by scanning this map sequentially and serving the first matching
 * entry.  This is mostly copied from the percpu_modalloc() allocator.
 * Chunks can be determined from the address using the index field
 * in the page struct. The index field contains a pointer to the chunk.
 *
 * To use this allocator, arch code should do the followings.
 *
 * - drop CONFIG_HAVE_LEGACY_PER_CPU_AREA
 *
 * - define __addr_to_pcpu_ptr() and __pcpu_ptr_to_addr() to translate
 *   regular address to percpu pointer and back if they need to be
 *   different from the default
 *
 * - use pcpu_setup_first_chunk() during percpu area initialization to
 *   setup the first chunk containing the kernel static percpu area
 */

#include <linux/bitmap.h>
#include <linux/bootmem.h>
#include <linux/list.h>
#include <linux/mm.h>
#include <linux/module.h>
#include <linux/mutex.h>
#include <linux/percpu.h>
#include <linux/pfn.h>
#include <linux/slab.h>
#include <linux/spinlock.h>
#include <linux/vmalloc.h>
#include <linux/workqueue.h>

#include <asm/cacheflush.h>
#include <asm/sections.h>
#include <asm/tlbflush.h>

#define PCPU_SLOT_BASE_SHIFT            5       /* 1-31 shares the same slot */
#define PCPU_DFL_MAP_ALLOC              16      /* start a map with 16 ents */

/* default addr <-> pcpu_ptr mapping, override in asm/percpu.h if necessary */
#ifndef __addr_to_pcpu_ptr
#define __addr_to_pcpu_ptr(addr)                                        \
        (void *)((unsigned long)(addr) - (unsigned long)pcpu_base_addr  \
                 + (unsigned long)__per_cpu_start)
#endif
#ifndef __pcpu_ptr_to_addr
#define __pcpu_ptr_to_addr(ptr)                                         \
        (void *)((unsigned long)(ptr) + (unsigned long)pcpu_base_addr   \
                 - (unsigned long)__per_cpu_start)
#endif

struct pcpu_chunk {
        struct list_head        list;           /* linked to pcpu_slot lists */
        int                     free_size;      /* free bytes in the chunk */
        int                     contig_hint;    /* max contiguous size hint */
        struct vm_struct        *vm;            /* mapped vmalloc region */
        int                     map_used;       /* # of map entries used */
        int                     map_alloc;      /* # of map entries allocated */
        int                     *map;           /* allocation map */
        bool                    immutable;      /* no [de]population allowed */
        struct page             **page;         /* points to page array */
        struct page             *page_ar[];     /* #cpus * UNIT_PAGES */
};

static int pcpu_unit_pages __read_mostly;
static int pcpu_unit_size __read_mostly;
static int pcpu_chunk_size __read_mostly;
static int pcpu_nr_slots __read_mostly;
static size_t pcpu_chunk_struct_size __read_mostly;

/* the address of the first chunk which starts with the kernel static area */
void *pcpu_base_addr __read_mostly;
EXPORT_SYMBOL_GPL(pcpu_base_addr);

/*
 * The first chunk which always exists.  Note that unlike other
 * chunks, this one can be allocated and mapped in several different
 * ways and thus often doesn't live in the vmalloc area.
 */
static struct pcpu_chunk *pcpu_first_chunk;

/*
 * Optional reserved chunk.  This chunk reserves part of the first
 * chunk and serves it for reserved allocations.  The amount of
 * reserved offset is in pcpu_reserved_chunk_limit.  When reserved
 * area doesn't exist, the following variables contain NULL and 0
 * respectively.
 */
static struct pcpu_chunk *pcpu_reserved_chunk;
static int pcpu_reserved_chunk_limit;

/*
 * Synchronization rules.
 *
 * There are two locks - pcpu_alloc_mutex and pcpu_lock.  The former
 * protects allocation/reclaim paths, chunks and chunk->page arrays.
 * The latter is a spinlock and protects the index data structures -
 * chunk slots, chunks and area maps in chunks.
 *
 * During allocation, pcpu_alloc_mutex is kept locked all the time and
 * pcpu_lock is grabbed and released as necessary.  All actual memory
 * allocations are done using GFP_KERNEL with pcpu_lock released.
 *
 * Free path accesses and alters only the index data structures, so it
 * can be safely called from atomic context.  When memory needs to be
 * returned to the system, free path schedules reclaim_work which
 * grabs both pcpu_alloc_mutex and pcpu_lock, unlinks chunks to be
 * reclaimed, release both locks and frees the chunks.  Note that it's
 * necessary to grab both locks to remove a chunk from circulation as
 * allocation path might be referencing the chunk with only
 * pcpu_alloc_mutex locked.
 */
static DEFINE_MUTEX(pcpu_alloc_mutex);  /* protects whole alloc and reclaim */
static DEFINE_SPINLOCK(pcpu_lock);      /* protects index data structures */

static struct list_head *pcpu_slot __read_mostly; /* chunk list slots */

/* reclaim work to release fully free chunks, scheduled from free path */
static void pcpu_reclaim(struct work_struct *work);
static DECLARE_WORK(pcpu_reclaim_work, pcpu_reclaim);

static int __pcpu_size_to_slot(int size)
{
        int highbit = fls(size);        /* size is in bytes */
        return max(highbit - PCPU_SLOT_BASE_SHIFT + 2, 1);
}

static int pcpu_size_to_slot(int size)
{
        if (size == pcpu_unit_size)
                return pcpu_nr_slots - 1;
        return __pcpu_size_to_slot(size);
}

static int pcpu_chunk_slot(const struct pcpu_chunk *chunk)
{
        if (chunk->free_size < sizeof(int) || chunk->contig_hint < sizeof(int))
                return 0;

        return pcpu_size_to_slot(chunk->free_size);
}

static int pcpu_page_idx(unsigned int cpu, int page_idx)
{
        return cpu * pcpu_unit_pages + page_idx;
}

static struct page **pcpu_chunk_pagep(struct pcpu_chunk *chunk,
                                      unsigned int cpu, int page_idx)
{
        return &chunk->page[pcpu_page_idx(cpu, page_idx)];
}

static unsigned long pcpu_chunk_addr(struct pcpu_chunk *chunk,
                                     unsigned int cpu, int page_idx)
{
        return (unsigned long)chunk->vm->addr +
                (pcpu_page_idx(cpu, page_idx) << PAGE_SHIFT);
}

static bool pcpu_chunk_page_occupied(struct pcpu_chunk *chunk,
                                     int page_idx)
{
        return *pcpu_chunk_pagep(chunk, 0, page_idx) != NULL;
}

/* set the pointer to a chunk in a page struct */
static void pcpu_set_page_chunk(struct page *page, struct pcpu_chunk *pcpu)
{
        page->index = (unsigned long)pcpu;
}

/* obtain pointer to a chunk from a page struct */
static struct pcpu_chunk *pcpu_get_page_chunk(struct page *page)
{
        return (struct pcpu_chunk *)page->index;
}

/**
 * pcpu_mem_alloc - allocate memory
 * @size: bytes to allocate
 *
 * Allocate @size bytes.  If @size is smaller than PAGE_SIZE,
 * kzalloc() is used; otherwise, vmalloc() is used.  The returned
 * memory is always zeroed.
 *
 * CONTEXT:
 * Does GFP_KERNEL allocation.
 *
 * RETURNS:
 * Pointer to the allocated area on success, NULL on failure.
 */
static void *pcpu_mem_alloc(size_t size)
{
        if (size <= PAGE_SIZE)
                return kzalloc(size, GFP_KERNEL);
        else {
                void *ptr = vmalloc(size);
                if (ptr)
                        memset(ptr, 0, size);
                return ptr;
        }
}

/**
 * pcpu_mem_free - free memory
 * @ptr: memory to free
 * @size: size of the area
 *
 * Free @ptr.  @ptr should have been allocated using pcpu_mem_alloc().
 */
static void pcpu_mem_free(void *ptr, size_t size)
{
        if (size <= PAGE_SIZE)
                kfree(ptr);
        else
                vfree(ptr);
}

/**
 * pcpu_chunk_relocate - put chunk in the appropriate chunk slot
 * @chunk: chunk of interest
 * @oslot: the previous slot it was on
 *
 * This function is called after an allocation or free changed @chunk.
 * New slot according to the changed state is determined and @chunk is
 * moved to the slot.  Note that the reserved chunk is never put on
 * chunk slots.
 *
 * CONTEXT:
 * pcpu_lock.
 */
static void pcpu_chunk_relocate(struct pcpu_chunk *chunk, int oslot)
{
        int nslot = pcpu_chunk_slot(chunk);

        if (chunk != pcpu_reserved_chunk && oslot != nslot) {
                if (oslot < nslot)
                        list_move(&chunk->list, &pcpu_slot[nslot]);
                else
                        list_move_tail(&chunk->list, &pcpu_slot[nslot]);
        }
}

/**
 * pcpu_chunk_addr_search - determine chunk containing specified address
 * @addr: address for which the chunk needs to be determined.
 *
 * RETURNS:
 * The address of the found chunk.
 */
static struct pcpu_chunk *pcpu_chunk_addr_search(void *addr)
{
        void *first_start = pcpu_first_chunk->vm->addr;

        /* is it in the first chunk? */
        if (addr >= first_start && addr < first_start + pcpu_unit_size) {
                /* is it in the reserved area? */
                if (addr < first_start + pcpu_reserved_chunk_limit)
                        return pcpu_reserved_chunk;
                return pcpu_first_chunk;
        }

        return pcpu_get_page_chunk(vmalloc_to_page(addr));
}

/**
 * pcpu_extend_area_map - extend area map for allocation
 * @chunk: target chunk
 *
 * Extend area map of @chunk so that it can accomodate an allocation.
 * A single allocation can split an area into three areas, so this
 * function makes sure that @chunk->map has at least two extra slots.
 *
 * CONTEXT:
 * pcpu_alloc_mutex, pcpu_lock.  pcpu_lock is released and reacquired
 * if area map is extended.
 *
 * RETURNS:
 * 0 if noop, 1 if successfully extended, -errno on failure.
 */
static int pcpu_extend_area_map(struct pcpu_chunk *chunk)
{
        int new_alloc;
        int *new;
        size_t size;

        /* has enough? */
        if (chunk->map_alloc >= chunk->map_used + 2)
                return 0;

        spin_unlock_irq(&pcpu_lock);

        new_alloc = PCPU_DFL_MAP_ALLOC;
        while (new_alloc < chunk->map_used + 2)
                new_alloc *= 2;

        new = pcpu_mem_alloc(new_alloc * sizeof(new[0]));
        if (!new) {
                spin_lock_irq(&pcpu_lock);
                return -ENOMEM;
        }

        /*
         * Acquire pcpu_lock and switch to new area map.  Only free
         * could have happened inbetween, so map_used couldn't have
         * grown.
         */
        spin_lock_irq(&pcpu_lock);
        BUG_ON(new_alloc < chunk->map_used + 2);

        size = chunk->map_alloc * sizeof(chunk->map[0]);
        memcpy(new, chunk->map, size);

        /*
         * map_alloc < PCPU_DFL_MAP_ALLOC indicates that the chunk is
         * one of the first chunks and still using static map.
         */
        if (chunk->map_alloc >= PCPU_DFL_MAP_ALLOC)
                pcpu_mem_free(chunk->map, size);

        chunk->map_alloc = new_alloc;
        chunk->map = new;
        return 0;
}

/**
 * pcpu_split_block - split a map block
 * @chunk: chunk of interest
 * @i: index of map block to split
 * @head: head size in bytes (can be 0)
 * @tail: tail size in bytes (can be 0)
 *
 * Split the @i'th map block into two or three blocks.  If @head is
 * non-zero, @head bytes block is inserted before block @i moving it
 * to @i+1 and reducing its size by @head bytes.
 *
 * If @tail is non-zero, the target block, which can be @i or @i+1
 * depending on @head, is reduced by @tail bytes and @tail byte block
 * is inserted after the target block.
 *
 * @chunk->map must have enough free slots to accomodate the split.
 *
 * CONTEXT:
 * pcpu_lock.
 */
static void pcpu_split_block(struct pcpu_chunk *chunk, int i,
                             int head, int tail)
{
        int nr_extra = !!head + !!tail;

        BUG_ON(chunk->map_alloc < chunk->map_used + nr_extra);

        /* insert new subblocks */
        memmove(&chunk->map[i + nr_extra], &chunk->map[i],
                sizeof(chunk->map[0]) * (chunk->map_used - i));
        chunk->map_used += nr_extra;

        if (head) {
                chunk->map[i + 1] = chunk->map[i] - head;
                chunk->map[i++] = head;
        }
        if (tail) {
                chunk->map[i++] -= tail;
                chunk->map[i] = tail;
        }
}

/**
 * pcpu_alloc_area - allocate area from a pcpu_chunk
 * @chunk: chunk of interest
 * @size: wanted size in bytes
 * @align: wanted align
 *
 * Try to allocate @size bytes area aligned at @align from @chunk.
 * Note that this function only allocates the offset.  It doesn't
 * populate or map the area.
 *
 * @chunk->map must have at least two free slots.
 *
 * CONTEXT:
 * pcpu_lock.
 *
 * RETURNS:
 * Allocated offset in @chunk on success, -1 if no matching area is
 * found.
 */
static int pcpu_alloc_area(struct pcpu_chunk *chunk, int size, int align)
{
        int oslot = pcpu_chunk_slot(chunk);
        int max_contig = 0;
        int i, off;

        for (i = 0, off = 0; i < chunk->map_used; off += abs(chunk->map[i++])) {
                bool is_last = i + 1 == chunk->map_used;
                int head, tail;

                /* extra for alignment requirement */
                head = ALIGN(off, align) - off;
                BUG_ON(i == 0 && head != 0);

                if (chunk->map[i] < 0)
                        continue;
                if (chunk->map[i] < head + size) {
                        max_contig = max(chunk->map[i], max_contig);
                        continue;
                }

                /*
                 * If head is small or the previous block is free,
                 * merge'em.  Note that 'small' is defined as smaller
                 * than sizeof(int), which is very small but isn't too
                 * uncommon for percpu allocations.
                 */
                if (head && (head < sizeof(int) || chunk->map[i - 1] > 0)) {
                        if (chunk->map[i - 1] > 0)
                                chunk->map[i - 1] += head;
                        else {
                                chunk->map[i - 1] -= head;
                                chunk->free_size -= head;
                        }
                        chunk->map[i] -= head;
                        off += head;
                        head = 0;
                }

                /* if tail is small, just keep it around */
                tail = chunk->map[i] - head - size;
                if (tail < sizeof(int))
                        tail = 0;

                /* split if warranted */
                if (head || tail) {
                        pcpu_split_block(chunk, i, head, tail);
                        if (head) {
                                i++;
                                off += head;
                                max_contig = max(chunk->map[i - 1], max_contig);
                        }
                        if (tail)
                                max_contig = max(chunk->map[i + 1], max_contig);
                }

                /* update hint and mark allocated */
                if (is_last)
                        chunk->contig_hint = max_contig; /* fully scanned */
                else
                        chunk->contig_hint = max(chunk->contig_hint,
                                                 max_contig);

                chunk->free_size -= chunk->map[i];
                chunk->map[i] = -chunk->map[i];

                pcpu_chunk_relocate(chunk, oslot);
                return off;
        }

        chunk->contig_hint = max_contig;        /* fully scanned */
        pcpu_chunk_relocate(chunk, oslot);

        /* tell the upper layer that this chunk has no matching area */
        return -1;
}

/**
 * pcpu_free_area - free area to a pcpu_chunk
 * @chunk: chunk of interest
 * @freeme: offset of area to free
 *
 * Free area starting from @freeme to @chunk.  Note that this function
 * only modifies the allocation map.  It doesn't depopulate or unmap
 * the area.
 *
 * CONTEXT:
 * pcpu_lock.
 */
static void pcpu_free_area(struct pcpu_chunk *chunk, int freeme)
{
        int oslot = pcpu_chunk_slot(chunk);
        int i, off;

        for (i = 0, off = 0; i < chunk->map_used; off += abs(chunk->map[i++]))
                if (off == freeme)
                        break;
        BUG_ON(off != freeme);
        BUG_ON(chunk->map[i] > 0);

        chunk->map[i] = -chunk->map[i];
        chunk->free_size += chunk->map[i];

        /* merge with previous? */
        if (i > 0 && chunk->map[i - 1] >= 0) {
                chunk->map[i - 1] += chunk->map[i];
                chunk->map_used--;
                memmove(&chunk->map[i], &chunk->map[i + 1],
                        (chunk->map_used - i) * sizeof(chunk->map[0]));
                i--;
        }
        /* merge with next? */
        if (i + 1 < chunk->map_used && chunk->map[i + 1] >= 0) {
                chunk->map[i] += chunk->map[i + 1];
                chunk->map_used--;
                memmove(&chunk->map[i + 1], &chunk->map[i + 2],
                        (chunk->map_used - (i + 1)) * sizeof(chunk->map[0]));
        }

        chunk->contig_hint = max(chunk->map[i], chunk->contig_hint);
        pcpu_chunk_relocate(chunk, oslot);
}

/**
 * pcpu_unmap - unmap pages out of a pcpu_chunk
 * @chunk: chunk of interest
 * @page_start: page index of the first page to unmap
 * @page_end: page index of the last page to unmap + 1
 * @flush_tlb: whether to flush tlb or not
 *
 * For each cpu, unmap pages [@page_start,@page_end) out of @chunk.
 * If @flush is true, vcache is flushed before unmapping and tlb
 * after.
 */
static void pcpu_unmap(struct pcpu_chunk *chunk, int page_start, int page_end,
                       bool flush_tlb)
{
        unsigned int last = num_possible_cpus() - 1;
        unsigned int cpu;

        /* unmap must not be done on immutable chunk */
        WARN_ON(chunk->immutable);

        /*
         * Each flushing trial can be very expensive, issue flush on
         * the whole region at once rather than doing it for each cpu.
         * This could be an overkill but is more scalable.
         */
        flush_cache_vunmap(pcpu_chunk_addr(chunk, 0, page_start),
                           pcpu_chunk_addr(chunk, last, page_end));

        for_each_possible_cpu(cpu)
                unmap_kernel_range_noflush(
                                pcpu_chunk_addr(chunk, cpu, page_start),
                                (page_end - page_start) << PAGE_SHIFT);

        /* ditto as flush_cache_vunmap() */
        if (flush_tlb)
                flush_tlb_kernel_range(pcpu_chunk_addr(chunk, 0, page_start),
                                       pcpu_chunk_addr(chunk, last, page_end));
}

/**
 * pcpu_depopulate_chunk - depopulate and unmap an area of a pcpu_chunk
 * @chunk: chunk to depopulate
 * @off: offset to the area to depopulate
 * @size: size of the area to depopulate in bytes
 * @flush: whether to flush cache and tlb or not
 *
 * For each cpu, depopulate and unmap pages [@page_start,@page_end)
 * from @chunk.  If @flush is true, vcache is flushed before unmapping
 * and tlb after.
 *
 * CONTEXT:
 * pcpu_alloc_mutex.
 */
static void pcpu_depopulate_chunk(struct pcpu_chunk *chunk, int off, int size,
                                  bool flush)
{
        int page_start = PFN_DOWN(off);
        int page_end = PFN_UP(off + size);
        int unmap_start = -1;
        int uninitialized_var(unmap_end);
        unsigned int cpu;
        int i;

        for (i = page_start; i < page_end; i++) {
                for_each_possible_cpu(cpu) {
                        struct page **pagep = pcpu_chunk_pagep(chunk, cpu, i);

                        if (!*pagep)
                                continue;

                        __free_page(*pagep);

                        /*
                         * If it's partial depopulation, it might get
                         * populated or depopulated again.  Mark the
                         * page gone.
                         */
                        *pagep = NULL;

                        unmap_start = unmap_start < 0 ? i : unmap_start;
                        unmap_end = i + 1;
                }
        }

        if (unmap_start >= 0)
                pcpu_unmap(chunk, unmap_start, unmap_end, flush);
}

static int __pcpu_map_pages(unsigned long addr, struct page **pages,
                            int nr_pages)
{
        return map_kernel_range_noflush(addr, nr_pages << PAGE_SHIFT,
                                        PAGE_KERNEL, pages);
}

/**
 * pcpu_map - map pages into a pcpu_chunk
 * @chunk: chunk of interest
 * @page_start: page index of the first page to map
 * @page_end: page index of the last page to map + 1
 *
 * For each cpu, map pages [@page_start,@page_end) into @chunk.
 * vcache is flushed afterwards.
 */
static int pcpu_map(struct pcpu_chunk *chunk, int page_start, int page_end)
{
        unsigned int last = num_possible_cpus() - 1;
        unsigned int cpu;
        int err;

        /* map must not be done on immutable chunk */
        WARN_ON(chunk->immutable);

        for_each_possible_cpu(cpu) {
                err = __pcpu_map_pages(pcpu_chunk_addr(chunk, cpu, page_start),
                                       pcpu_chunk_pagep(chunk, cpu, page_start),
                                       page_end - page_start);
                if (err < 0)
                        return err;
        }

        /* flush at once, please read comments in pcpu_unmap() */
        flush_cache_vmap(pcpu_chunk_addr(chunk, 0, page_start),
                         pcpu_chunk_addr(chunk, last, page_end));
        return 0;
}

/**
 * pcpu_populate_chunk - populate and map an area of a pcpu_chunk
 * @chunk: chunk of interest
 * @off: offset to the area to populate
 * @size: size of the area to populate in bytes
 *
 * For each cpu, populate and map pages [@page_start,@page_end) into
 * @chunk.  The area is cleared on return.
 *
 * CONTEXT:
 * pcpu_alloc_mutex, does GFP_KERNEL allocation.
 */
static int pcpu_populate_chunk(struct pcpu_chunk *chunk, int off, int size)
{
        const gfp_t alloc_mask = GFP_KERNEL | __GFP_HIGHMEM | __GFP_COLD;
        int page_start = PFN_DOWN(off);
        int page_end = PFN_UP(off + size);
        int map_start = -1;
        int uninitialized_var(map_end);
        unsigned int cpu;
        int i;

        for (i = page_start; i < page_end; i++) {
                if (pcpu_chunk_page_occupied(chunk, i)) {
                        if (map_start >= 0) {
                                if (pcpu_map(chunk, map_start, map_end))
                                        goto err;
                                map_start = -1;
                        }
                        continue;
                }

                map_start = map_start < 0 ? i : map_start;
                map_end = i + 1;

                for_each_possible_cpu(cpu) {
                        struct page **pagep = pcpu_chunk_pagep(chunk, cpu, i);

                        *pagep = alloc_pages_node(cpu_to_node(cpu),
                                                  alloc_mask, 0);
                        if (!*pagep)
                                goto err;
                        pcpu_set_page_chunk(*pagep, chunk);
                }
        }

        if (map_start >= 0 && pcpu_map(chunk, map_start, map_end))
                goto err;

        for_each_possible_cpu(cpu)
                memset(chunk->vm->addr + cpu * pcpu_unit_size + off, 0,
                       size);

        return 0;
err:
        /* likely under heavy memory pressure, give memory back */
        pcpu_depopulate_chunk(chunk, off, size, true);
        return -ENOMEM;
}

static void free_pcpu_chunk(struct pcpu_chunk *chunk)
{
        if (!chunk)
                return;
        if (chunk->vm)
                free_vm_area(chunk->vm);
        pcpu_mem_free(chunk->map, chunk->map_alloc * sizeof(chunk->map[0]));
        kfree(chunk);
}

static struct pcpu_chunk *alloc_pcpu_chunk(void)
{
        struct pcpu_chunk *chunk;

        chunk = kzalloc(pcpu_chunk_struct_size, GFP_KERNEL);
        if (!chunk)
                return NULL;

        chunk->map = pcpu_mem_alloc(PCPU_DFL_MAP_ALLOC * sizeof(chunk->map[0]));
        chunk->map_alloc = PCPU_DFL_MAP_ALLOC;
        chunk->map[chunk->map_used++] = pcpu_unit_size;
        chunk->page = chunk->page_ar;

        chunk->vm = get_vm_area(pcpu_chunk_size, GFP_KERNEL);
        if (!chunk->vm) {
                free_pcpu_chunk(chunk);
                return NULL;
        }

        INIT_LIST_HEAD(&chunk->list);
        chunk->free_size = pcpu_unit_size;
        chunk->contig_hint = pcpu_unit_size;

        return chunk;
}

/**
 * pcpu_alloc - the percpu allocator
 * @size: size of area to allocate in bytes
 * @align: alignment of area (max PAGE_SIZE)
 * @reserved: allocate from the reserved chunk if available
 *
 * Allocate percpu area of @size bytes aligned at @align.
 *
 * CONTEXT:
 * Does GFP_KERNEL allocation.
 *
 * RETURNS:
 * Percpu pointer to the allocated area on success, NULL on failure.
 */
static void *pcpu_alloc(size_t size, size_t align, bool reserved)
{
        struct pcpu_chunk *chunk;
        int slot, off;

        if (unlikely(!size || size > PCPU_MIN_UNIT_SIZE || align > PAGE_SIZE)) {
                WARN(true, "illegal size (%zu) or align (%zu) for "
                     "percpu allocation\n", size, align);
                return NULL;
        }

        mutex_lock(&pcpu_alloc_mutex);
        spin_lock_irq(&pcpu_lock);

        /* serve reserved allocations from the reserved chunk if available */
        if (reserved && pcpu_reserved_chunk) {
                chunk = pcpu_reserved_chunk;
                if (size > chunk->contig_hint ||
                    pcpu_extend_area_map(chunk) < 0)
                        goto fail_unlock;
                off = pcpu_alloc_area(chunk, size, align);
                if (off >= 0)
                        goto area_found;
                goto fail_unlock;
        }

restart:
        /* search through normal chunks */
        for (slot = pcpu_size_to_slot(size); slot < pcpu_nr_slots; slot++) {
                list_for_each_entry(chunk, &pcpu_slot[slot], list) {
                        if (size > chunk->contig_hint)
                                continue;

                        switch (pcpu_extend_area_map(chunk)) {
                        case 0:
                                break;
                        case 1:
                                goto restart;   /* pcpu_lock dropped, restart */
                        default:
                                goto fail_unlock;
                        }

                        off = pcpu_alloc_area(chunk, size, align);
                        if (off >= 0)
                                goto area_found;
                }
        }

        /* hmmm... no space left, create a new chunk */
        spin_unlock_irq(&pcpu_lock);

        chunk = alloc_pcpu_chunk();
        if (!chunk)
                goto fail_unlock_mutex;

        spin_lock_irq(&pcpu_lock);
        pcpu_chunk_relocate(chunk, -1);
        goto restart;

area_found:
        spin_unlock_irq(&pcpu_lock);

        /* populate, map and clear the area */
        if (pcpu_populate_chunk(chunk, off, size)) {
                spin_lock_irq(&pcpu_lock);
                pcpu_free_area(chunk, off);
                goto fail_unlock;
        }

        mutex_unlock(&pcpu_alloc_mutex);

        return __addr_to_pcpu_ptr(chunk->vm->addr + off);

fail_unlock:
        spin_unlock_irq(&pcpu_lock);
fail_unlock_mutex:
        mutex_unlock(&pcpu_alloc_mutex);
        return NULL;
}

/**
 * __alloc_percpu - allocate dynamic percpu area
 * @size: size of area to allocate in bytes
 * @align: alignment of area (max PAGE_SIZE)
 *
 * Allocate percpu area of @size bytes aligned at @align.  Might
 * sleep.  Might trigger writeouts.
 *
 * CONTEXT:
 * Does GFP_KERNEL allocation.
 *
 * RETURNS:
 * Percpu pointer to the allocated area on success, NULL on failure.
 */
void *__alloc_percpu(size_t size, size_t align)
{
        return pcpu_alloc(size, align, false);
}
EXPORT_SYMBOL_GPL(__alloc_percpu);

/**
 * __alloc_reserved_percpu - allocate reserved percpu area
 * @size: size of area to allocate in bytes
 * @align: alignment of area (max PAGE_SIZE)
 *
 * Allocate percpu area of @size bytes aligned at @align from reserved
 * percpu area if arch has set it up; otherwise, allocation is served
 * from the same dynamic area.  Might sleep.  Might trigger writeouts.
 *
 * CONTEXT:
 * Does GFP_KERNEL allocation.
 *
 * RETURNS:
 * Percpu pointer to the allocated area on success, NULL on failure.
 */
void *__alloc_reserved_percpu(size_t size, size_t align)
{
        return pcpu_alloc(size, align, true);
}

/**
 * pcpu_reclaim - reclaim fully free chunks, workqueue function
 * @work: unused
 *
 * Reclaim all fully free chunks except for the first one.
 *
 * CONTEXT:
 * workqueue context.
 */
static void pcpu_reclaim(struct work_struct *work)
{
        LIST_HEAD(todo);
        struct list_head *head = &pcpu_slot[pcpu_nr_slots - 1];
        struct pcpu_chunk *chunk, *next;

        mutex_lock(&pcpu_alloc_mutex);
        spin_lock_irq(&pcpu_lock);

        list_for_each_entry_safe(chunk, next, head, list) {
                WARN_ON(chunk->immutable);

                /* spare the first one */
                if (chunk == list_first_entry(head, struct pcpu_chunk, list))
                        continue;

                list_move(&chunk->list, &todo);
        }

        spin_unlock_irq(&pcpu_lock);
        mutex_unlock(&pcpu_alloc_mutex);

        list_for_each_entry_safe(chunk, next, &todo, list) {
                pcpu_depopulate_chunk(chunk, 0, pcpu_unit_size, false);
                free_pcpu_chunk(chunk);
        }
}

/**
 * free_percpu - free percpu area
 * @ptr: pointer to area to free
 *
 * Free percpu area @ptr.
 *
 * CONTEXT:
 * Can be called from atomic context.
 */
void free_percpu(void *ptr)
{
        void *addr = __pcpu_ptr_to_addr(ptr);
        struct pcpu_chunk *chunk;
        unsigned long flags;
        int off;

        if (!ptr)
                return;

        spin_lock_irqsave(&pcpu_lock, flags);

        chunk = pcpu_chunk_addr_search(addr);
        off = addr - chunk->vm->addr;

        pcpu_free_area(chunk, off);

        /* if there are more than one fully free chunks, wake up grim reaper */
        if (chunk->free_size == pcpu_unit_size) {
                struct pcpu_chunk *pos;

                list_for_each_entry(pos, &pcpu_slot[pcpu_nr_slots - 1], list)
                        if (pos != chunk) {
                                schedule_work(&pcpu_reclaim_work);
                                break;
                        }
        }

        spin_unlock_irqrestore(&pcpu_lock, flags);
}
EXPORT_SYMBOL_GPL(free_percpu);

/**
 * pcpu_setup_first_chunk - initialize the first percpu chunk
 * @get_page_fn: callback to fetch page pointer
 * @static_size: the size of static percpu area in bytes
 * @reserved_size: the size of reserved percpu area in bytes
 * @dyn_size: free size for dynamic allocation in bytes, -1 for auto
 * @unit_size: unit size in bytes, must be multiple of PAGE_SIZE, -1 for auto
 * @base_addr: mapped address, NULL for auto
 * @populate_pte_fn: callback to allocate pagetable, NULL if unnecessary
 *
 * Initialize the first percpu chunk which contains the kernel static
 * perpcu area.  This function is to be called from arch percpu area
 * setup path.  The first two parameters are mandatory.  The rest are
 * optional.
 *
 * @get_page_fn() should return pointer to percpu page given cpu
 * number and page number.  It should at least return enough pages to
 * cover the static area.  The returned pages for static area should
 * have been initialized with valid data.  If @unit_size is specified,
 * it can also return pages after the static area.  NULL return
 * indicates end of pages for the cpu.  Note that @get_page_fn() must
 * return the same number of pages for all cpus.
 *
 * @reserved_size, if non-zero, specifies the amount of bytes to
 * reserve after the static area in the first chunk.  This reserves
 * the first chunk such that it's available only through reserved
 * percpu allocation.  This is primarily used to serve module percpu
 * static areas on architectures where the addressing model has
 * limited offset range for symbol relocations to guarantee module
 * percpu symbols fall inside the relocatable range.
 *
 * @dyn_size, if non-negative, determines the number of bytes
 * available for dynamic allocation in the first chunk.  Specifying
 * non-negative value makes percpu leave alone the area beyond
 * @static_size + @reserved_size + @dyn_size.
 *
 * @unit_size, if non-negative, specifies unit size and must be
 * aligned to PAGE_SIZE and equal to or larger than @static_size +
 * @reserved_size + if non-negative, @dyn_size.
 *
 * Non-null @base_addr means that the caller already allocated virtual
 * region for the first chunk and mapped it.  percpu must not mess
 * with the chunk.  Note that @base_addr with 0 @unit_size or non-NULL
 * @populate_pte_fn doesn't make any sense.
 *
 * @populate_pte_fn is used to populate the pagetable.  NULL means the
 * caller already populated the pagetable.
 *
 * If the first chunk ends up with both reserved and dynamic areas, it
 * is served by two chunks - one to serve the core static and reserved
 * areas and the other for the dynamic area.  They share the same vm
 * and page map but uses different area allocation map to stay away
 * from each other.  The latter chunk is circulated in the chunk slots
 * and available for dynamic allocation like any other chunks.
 *
 * RETURNS:
 * The determined pcpu_unit_size which can be used to initialize
 * percpu access.
 */
size_t __init pcpu_setup_first_chunk(pcpu_get_page_fn_t get_page_fn,
                                     size_t static_size, size_t reserved_size,
                                     ssize_t dyn_size, ssize_t unit_size,
                                     void *base_addr,
                                     pcpu_fc_populate_pte_fn_t populate_pte_fn)
{
        static struct vm_struct first_vm;
        static int smap[2], dmap[2];
        size_t size_sum = static_size + reserved_size +
                          (dyn_size >= 0 ? dyn_size : 0);
        struct pcpu_chunk *schunk, *dchunk = NULL;
        unsigned int cpu;
        int nr_pages;
        int err, i;

        /* santiy checks */
        BUILD_BUG_ON(ARRAY_SIZE(smap) >= PCPU_DFL_MAP_ALLOC ||
                     ARRAY_SIZE(dmap) >= PCPU_DFL_MAP_ALLOC);
        BUG_ON(!static_size);
        if (unit_size >= 0) {
                BUG_ON(unit_size < size_sum);
                BUG_ON(unit_size & ~PAGE_MASK);
                BUG_ON(unit_size < PCPU_MIN_UNIT_SIZE);
        } else
                BUG_ON(base_addr);
        BUG_ON(base_addr && populate_pte_fn);

        if (unit_size >= 0)
                pcpu_unit_pages = unit_size >> PAGE_SHIFT;
        else
                pcpu_unit_pages = max_t(int, PCPU_MIN_UNIT_SIZE >> PAGE_SHIFT,
                                        PFN_UP(size_sum));

        pcpu_unit_size = pcpu_unit_pages << PAGE_SHIFT;
        pcpu_chunk_size = num_possible_cpus() * pcpu_unit_size;
        pcpu_chunk_struct_size = sizeof(struct pcpu_chunk)
                + num_possible_cpus() * pcpu_unit_pages * sizeof(struct page *);

        if (dyn_size < 0)
                dyn_size = pcpu_unit_size - static_size - reserved_size;

        /*
         * Allocate chunk slots.  The additional last slot is for
         * empty chunks.
         */
        pcpu_nr_slots = __pcpu_size_to_slot(pcpu_unit_size) + 2;
        pcpu_slot = alloc_bootmem(pcpu_nr_slots * sizeof(pcpu_slot[0]));
        for (i = 0; i < pcpu_nr_slots; i++)
                INIT_LIST_HEAD(&pcpu_slot[i]);

        /*
         * Initialize static chunk.  If reserved_size is zero, the
         * static chunk covers static area + dynamic allocation area
         * in the first chunk.  If reserved_size is not zero, it
         * covers static area + reserved area (mostly used for module
         * static percpu allocation).
         */
        schunk = alloc_bootmem(pcpu_chunk_struct_size);
        INIT_LIST_HEAD(&schunk->list);
        schunk->vm = &first_vm;
        schunk->map = smap;
        schunk->map_alloc = ARRAY_SIZE(smap);
        schunk->page = schunk->page_ar;

        if (reserved_size) {
                schunk->free_size = reserved_size;
                pcpu_reserved_chunk = schunk;
                pcpu_reserved_chunk_limit = static_size + reserved_size;
        } else {
                schunk->free_size = dyn_size;
                dyn_size = 0;                   /* dynamic area covered */
        }
        schunk->contig_hint = schunk->free_size;

        schunk->map[schunk->map_used++] = -static_size;
        if (schunk->free_size)
                schunk->map[schunk->map_used++] = schunk->free_size;

        /* init dynamic chunk if necessary */
        if (dyn_size) {
                dchunk = alloc_bootmem(sizeof(struct pcpu_chunk));
                INIT_LIST_HEAD(&dchunk->list);
                dchunk->vm = &first_vm;
                dchunk->map = dmap;
                dchunk->map_alloc = ARRAY_SIZE(dmap);
                dchunk->page = schunk->page_ar; /* share page map with schunk */

                dchunk->contig_hint = dchunk->free_size = dyn_size;
                dchunk->map[dchunk->map_used++] = -pcpu_reserved_chunk_limit;
                dchunk->map[dchunk->map_used++] = dchunk->free_size;
        }

        /* allocate vm address */
        first_vm.flags = VM_ALLOC;
        first_vm.size = pcpu_chunk_size;

        if (!base_addr)
                vm_area_register_early(&first_vm, PAGE_SIZE);
        else {
                /*
                 * Pages already mapped.  No need to remap into
                 * vmalloc area.  In this case the first chunks can't
                 * be mapped or unmapped by percpu and are marked
                 * immutable.
                 */
                first_vm.addr = base_addr;
                schunk->immutable = true;
                if (dchunk)
                        dchunk->immutable = true;
        }

        /* assign pages */
        nr_pages = -1;
        for_each_possible_cpu(cpu) {
                for (i = 0; i < pcpu_unit_pages; i++) {
                        struct page *page = get_page_fn(cpu, i);

                        if (!page)
                                break;
                        *pcpu_chunk_pagep(schunk, cpu, i) = page;
                }

                BUG_ON(i < PFN_UP(static_size));

                if (nr_pages < 0)
                        nr_pages = i;
                else
                        BUG_ON(nr_pages != i);
        }

        /* map them */
        if (populate_pte_fn) {
                for_each_possible_cpu(cpu)
                        for (i = 0; i < nr_pages; i++)
                                populate_pte_fn(pcpu_chunk_addr(schunk,
                                                                cpu, i));

                err = pcpu_map(schunk, 0, nr_pages);
                if (err)
                        panic("failed to setup static percpu area, err=%d\n",
                              err);
        }

        /* link the first chunk in */
        pcpu_first_chunk = dchunk ?: schunk;
        pcpu_chunk_relocate(pcpu_first_chunk, -1);

        /* we're done */
        pcpu_base_addr = (void *)pcpu_chunk_addr(schunk, 0, 0);
        return pcpu_unit_size;
}

/*
 * Embedding first chunk setup helper.
 */
static void *pcpue_ptr __initdata;
static size_t pcpue_size __initdata;
static size_t pcpue_unit_size __initdata;

static struct page * __init pcpue_get_page(unsigned int cpu, int pageno)
{
        size_t off = (size_t)pageno << PAGE_SHIFT;

        if (off >= pcpue_size)
                return NULL;

        return virt_to_page(pcpue_ptr + cpu * pcpue_unit_size + off);
}

/**
 * pcpu_embed_first_chunk - embed the first percpu chunk into bootmem
 * @static_size: the size of static percpu area in bytes
 * @reserved_size: the size of reserved percpu area in bytes
 * @dyn_size: free size for dynamic allocation in bytes, -1 for auto
 *
 * This is a helper to ease setting up embedded first percpu chunk and
 * can be called where pcpu_setup_first_chunk() is expected.
 *
 * If this function is used to setup the first chunk, it is allocated
 * as a contiguous area using bootmem allocator and used as-is without
 * being mapped into vmalloc area.  This enables the first chunk to
 * piggy back on the linear physical mapping which often uses larger
 * page size.
 *
 * When @dyn_size is positive, dynamic area might be larger than
 * specified to fill page alignment.  When @dyn_size is auto,
 * @dyn_size is just big enough to fill page alignment after static
 * and reserved areas.
 *
 * If the needed size is smaller than the minimum or specified unit
 * size, the leftover is returned to the bootmem allocator.
 *
 * RETURNS:
 * The determined pcpu_unit_size which can be used to initialize
 * percpu access on success, -errno on failure.
 */
ssize_t __init pcpu_embed_first_chunk(size_t static_size, size_t reserved_size,
                                      ssize_t dyn_size)
{
        size_t chunk_size;
        unsigned int cpu;

        /* determine parameters and allocate */
        pcpue_size = PFN_ALIGN(static_size + reserved_size +
                               (dyn_size >= 0 ? dyn_size : 0));
        if (dyn_size != 0)
                dyn_size = pcpue_size - static_size - reserved_size;

        pcpue_unit_size = max_t(size_t, pcpue_size, PCPU_MIN_UNIT_SIZE);
        chunk_size = pcpue_unit_size * num_possible_cpus();

        pcpue_ptr = __alloc_bootmem_nopanic(chunk_size, PAGE_SIZE,
                                            __pa(MAX_DMA_ADDRESS));
        if (!pcpue_ptr) {
                pr_warning("PERCPU: failed to allocate %zu bytes for "
                           "embedding\n", chunk_size);
                return -ENOMEM;
        }

        /* return the leftover and copy */
        for_each_possible_cpu(cpu) {
                void *ptr = pcpue_ptr + cpu * pcpue_unit_size;

                free_bootmem(__pa(ptr + pcpue_size),
                             pcpue_unit_size - pcpue_size);
                memcpy(ptr, __per_cpu_load, static_size);
        }

        /* we're ready, commit */
        pr_info("PERCPU: Embedded %zu pages at %p, static data %zu bytes\n",
                pcpue_size >> PAGE_SHIFT, pcpue_ptr, static_size);

        return pcpu_setup_first_chunk(pcpue_get_page, static_size,
                                      reserved_size, dyn_size,
                                      pcpue_unit_size, pcpue_ptr, NULL);
}

/*
 * 4k page first chunk setup helper.
 */
static struct page **pcpu4k_pages __initdata;
static int pcpu4k_unit_pages __initdata;

static struct page * __init pcpu4k_get_page(unsigned int cpu, int pageno)
{
        if (pageno < pcpu4k_unit_pages)
                return pcpu4k_pages[cpu * pcpu4k_unit_pages + pageno];
        return NULL;
}

/**
 * pcpu_4k_first_chunk - map the first chunk using PAGE_SIZE pages
 * @static_size: the size of static percpu area in bytes
 * @reserved_size: the size of reserved percpu area in bytes
 * @alloc_fn: function to allocate percpu page, always called with PAGE_SIZE
 * @free_fn: funtion to free percpu page, always called with PAGE_SIZE
 * @populate_pte_fn: function to populate pte
 *
 * This is a helper to ease setting up embedded first percpu chunk and
 * can be called where pcpu_setup_first_chunk() is expected.
 *
 * This is the basic allocator.  Static percpu area is allocated
 * page-by-page into vmalloc area.
 *
 * RETURNS:
 * The determined pcpu_unit_size which can be used to initialize
 * percpu access on success, -errno on failure.
 */
ssize_t __init pcpu_4k_first_chunk(size_t static_size, size_t reserved_size,
                                   pcpu_fc_alloc_fn_t alloc_fn,
                                   pcpu_fc_free_fn_t free_fn,
                                   pcpu_fc_populate_pte_fn_t populate_pte_fn)
{
        static struct vm_struct vm;
        size_t pages_size;
        unsigned int cpu;
        int i, j;
        ssize_t ret;

        pcpu4k_unit_pages = PFN_UP(max_t(size_t, static_size + reserved_size,
                                         PCPU_MIN_UNIT_SIZE));

        /* unaligned allocations can't be freed, round up to page size */
        pages_size = PFN_ALIGN(pcpu4k_unit_pages * num_possible_cpus() *
                               sizeof(pcpu4k_pages[0]));
        pcpu4k_pages = alloc_bootmem(pages_size);

        /* allocate pages */
        j = 0;
        for_each_possible_cpu(cpu)
                for (i = 0; i < pcpu4k_unit_pages; i++) {
                        void *ptr;

                        ptr = alloc_fn(cpu, PAGE_SIZE);
                        if (!ptr) {
                                pr_warning("PERCPU: failed to allocate "
                                           "4k page for cpu%u\n", cpu);
                                goto enomem;
                        }
                        pcpu4k_pages[j++] = virt_to_page(ptr);
                }

        /* allocate vm area, map the pages and copy static data */
        vm.flags = VM_ALLOC;
        vm.size = num_possible_cpus() * pcpu4k_unit_pages << PAGE_SHIFT;
        vm_area_register_early(&vm, PAGE_SIZE);

        for_each_possible_cpu(cpu) {
                unsigned long unit_addr = (unsigned long)vm.addr +
                        (cpu * pcpu4k_unit_pages << PAGE_SHIFT);

                for (i = 0; i < pcpu4k_unit_pages; i++)
                        populate_pte_fn(unit_addr + (i << PAGE_SHIFT));

                /* pte already populated, the following shouldn't fail */
                ret = __pcpu_map_pages(unit_addr,
                                       &pcpu4k_pages[cpu * pcpu4k_unit_pages],
                                       pcpu4k_unit_pages);
                if (ret < 0)
                        panic("failed to map percpu area, err=%zd\n", ret);

                /*
                 * FIXME: Archs with virtual cache should flush local
                 * cache for the linear mapping here - something
                 * equivalent to flush_cache_vmap() on the local cpu.
                 * flush_cache_vmap() can't be used as most supporting
                 * data structures are not set up yet.
                 */

                /* copy static data */
                memcpy((void *)unit_addr, __per_cpu_load, static_size);
        }

        /* we're ready, commit */
        pr_info("PERCPU: %d 4k pages per cpu, static data %zu bytes\n",
                pcpu4k_unit_pages, static_size);

        ret = pcpu_setup_first_chunk(pcpu4k_get_page, static_size,
                                     reserved_size, -1,
                                     pcpu4k_unit_pages << PAGE_SHIFT, vm.addr,
                                     NULL);
        goto out_free_ar;

enomem:
        while (--j >= 0)
                free_fn(page_address(pcpu4k_pages[j]), PAGE_SIZE);
        ret = -ENOMEM;
out_free_ar:
        free_bootmem(__pa(pcpu4k_pages), pages_size);
        return ret;
}

/*
 * Generic percpu area setup.
 *
 * The embedding helper is used because its behavior closely resembles
 * the original non-dynamic generic percpu area setup.  This is
 * important because many archs have addressing restrictions and might
 * fail if the percpu area is located far away from the previous
 * location.  As an added bonus, in non-NUMA cases, embedding is
 * generally a good idea TLB-wise because percpu area can piggy back
 * on the physical linear memory mapping which uses large page
 * mappings on applicable archs.
 */
#ifndef CONFIG_HAVE_SETUP_PER_CPU_AREA
unsigned long __per_cpu_offset[NR_CPUS] __read_mostly;
EXPORT_SYMBOL(__per_cpu_offset);

void __init setup_per_cpu_areas(void)
{
        size_t static_size = __per_cpu_end - __per_cpu_start;
        ssize_t unit_size;
        unsigned long delta;
        unsigned int cpu;

        /*
         * Always reserve area for module percpu variables.  That's
         * what the legacy allocator did.
         */
        unit_size = pcpu_embed_first_chunk(static_size, PERCPU_MODULE_RESERVE,
                                           PERCPU_DYNAMIC_RESERVE);
        if (unit_size < 0)
                panic("Failed to initialized percpu areas.");

        delta = (unsigned long)pcpu_base_addr - (unsigned long)__per_cpu_start;
        for_each_possible_cpu(cpu)
                __per_cpu_offset[cpu] = delta + cpu * unit_size;
}
#endif /* CONFIG_HAVE_SETUP_PER_CPU_AREA */