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>

1 files changed, 3 insertions, 0 deletions

diff --git a/include/linux/fscache-cache.h b/include/linux/fscache-cache.h
index 907bb56c5888..5db50002f3b5 100644
--- 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++;