CacheFiles: Don't write a full page if there's only a partial page to cache

cachefiles_write_page() writes a full page to the backing file for the last
page of the netfs file, even if the netfs file's last page is only a partial
page.

This causes the EOF on the backing file to be extended beyond the EOF of the
netfs, and thus the backing file will be truncated by cachefiles_attr_changed()
called from cachefiles_lookup_object().

So we need to limit the write we make to the backing file on that last page
such that it doesn't push the EOF too far.

Also, if a backing file that has a partial page at the end is expanded, we
discard the partial page and refetch it on the basis that we then have a hole
in the file with invalid data, and should the power go out...  A better way to
deal with this could be to record a note that the partial page contains invalid
data until the correct data is written into it.

This isn't a problem for netfs's that discard the whole backing file if the
file size changes (such as NFS).

Signed-off-by: David Howells <dhowells@redhat.com>

diff --git a/fs/cachefiles/interface.c b/fs/cachefiles/interface.c
index dd7f852..8e67abf 100644 (file)
--- a/fs/cachefiles/interface.c
+++ b/fs/cachefiles/interface.c
@@ -404,12 +404,26 @@ static int cachefiles_attr_changed(struct fscache_object *_object)
        if (oi_size == ni_size)
                return 0;
 
-       newattrs.ia_size = ni_size;
-       newattrs.ia_valid = ATTR_SIZE;
-
        cachefiles_begin_secure(cache, &saved_cred);
        mutex_lock(&object->backer->d_inode->i_mutex);
+
+       /* if there's an extension to a partial page at the end of the backing
+        * file, we need to discard the partial page so that we pick up new
+        * data after it */
+       if (oi_size & ~PAGE_MASK && ni_size > oi_size) {
+               _debug("discard tail %llx", oi_size);
+               newattrs.ia_valid = ATTR_SIZE;
+               newattrs.ia_size = oi_size & PAGE_MASK;
+               ret = notify_change(object->backer, &newattrs);
+               if (ret < 0)
+                       goto truncate_failed;
+       }
+
+       newattrs.ia_valid = ATTR_SIZE;
+       newattrs.ia_size = ni_size;
        ret = notify_change(object->backer, &newattrs);
+
+truncate_failed:
        mutex_unlock(&object->backer->d_inode->i_mutex);
        cachefiles_end_secure(cache, saved_cred);
 

diff --git a/fs/cachefiles/rdwr.c b/fs/cachefiles/rdwr.c
index 3304646..5a84fd7 100644 (file)
--- a/fs/cachefiles/rdwr.c
+++ b/fs/cachefiles/rdwr.c
@@ -803,7 +803,8 @@ int cachefiles_write_page(struct fscache_storage *op, struct page *page)
        struct cachefiles_cache *cache;
        mm_segment_t old_fs;
        struct file *file;
-       loff_t pos;
+       loff_t pos, eof;
+       size_t len;
        void *data;
        int ret;
 
@@ -837,15 +838,29 @@ int cachefiles_write_page(struct fscache_storage *op, struct page *page)
                ret = -EIO;
                if (file->f_op->write) {
                        pos = (loff_t) page->index << PAGE_SHIFT;
+
+                       /* we mustn't write more data than we have, so we have
+                        * to beware of a partial page at EOF */
+                       eof = object->fscache.store_limit_l;
+                       len = PAGE_SIZE;
+                       if (eof & ~PAGE_MASK) {
+                               ASSERTCMP(pos, <, eof);
+                               if (eof - pos < PAGE_SIZE) {
+                                       _debug("cut short %llx to %llx",
+                                              pos, eof);
+                                       len = eof - pos;
+                                       ASSERTCMP(pos + len, ==, eof);
+                               }
+                       }
+
                        data = kmap(page);
                        old_fs = get_fs();
                        set_fs(KERNEL_DS);
                        ret = file->f_op->write(
-                               file, (const void __user *) data, PAGE_SIZE,
-                               &pos);
+                               file, (const void __user *) data, len, &pos);
                        set_fs(old_fs);
                        kunmap(page);
-                       if (ret != PAGE_SIZE)
+                       if (ret != len)
                                ret = -EIO;
                }
                fput(file);

diff --git a/include/linux/fscache-cache.h b/include/linux/fscache-cache.h
index 907bb56..5db5000 100644 (file)
--- a/include/linux/fscache-cache.h
+++ b/include/linux/fscache-cache.h
@@ -395,6 +395,7 @@ struct fscache_object {
        struct rb_node          objlist_link;   /* link in global object list */
 #endif
        pgoff_t                 store_limit;    /* current storage limit */
+       loff_t                  store_limit_l;  /* current storage limit */
 };
 
 extern const char *fscache_object_states[];
@@ -439,6 +440,7 @@ void fscache_object_init(struct fscache_object *object,
        object->events = object->event_mask = 0;
        object->flags = 0;
        object->store_limit = 0;
+       object->store_limit_l = 0;
        object->cache = cache;
        object->cookie = cookie;
        object->parent = NULL;
@@ -491,6 +493,7 @@ static inline void fscache_object_lookup_error(struct fscache_object *object)
 static inline
 void fscache_set_store_limit(struct fscache_object *object, loff_t i_size)
 {
+       object->store_limit_l = i_size;
        object->store_limit = i_size >> PAGE_SHIFT;
        if (i_size & ~PAGE_MASK)
                object->store_limit++;