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, 17 insertions, 3 deletions

diff --git a/fs/cachefiles/interface.c b/fs/cachefiles/interface.c
index dd7f852..8e67abf 100644
--- 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);