[PATCH] qgroup: Prevent qgroup->reserved from going subzero

[Goldwyn Rodrigues <rgoldwyn@suse.de>]

From: Goldwyn Rodrigues <rgoldwyn@suse.com>

While free'ing qgroup->reserved resources, we must check
if the page is already commmitted to disk or still in memory.
If not, the reserve free is doubly accounted, once while
invalidating the page, and the next time while free'ing
delalloc. This results is qgroup->reserved(u64) going subzero,
thus very large value. So, no further I/O can be performed.

This is also expressed in the comments, but not performed.

Testcase:
SCRATCH_DEV=/dev/vdb
SCRATCH_MNT=/mnt
mkfs.btrfs -f $SCRATCH_DEV
mount -t btrfs $SCRATCH_DEV $SCRATCH_MNT
cd $SCRATCH_MNT
btrfs quota enable $SCRATCH_MNT
btrfs subvolume create a
btrfs qgroup limit 50m a $SCRATCH_MNT
sync
for c in {1..15}; do
dd if=/dev/zero  bs=1M count=40 of=$SCRATCH_MNT/a/file;
done

sleep 10
sync
sleep 5

touch $SCRATCH_MNT/a/newfile

echo "Removing file"
rm $SCRATCH_MNT/a/file

Fixes: b9d0b38928 ("btrfs: Add handler for invalidate page")
Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
---
 fs/btrfs/inode.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index e6811c4..2e2a026 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -8917,7 +8917,8 @@ again:
 	 * 2) Not written to disk
 	 *    This means the reserved space should be freed here.
 	 */
-	btrfs_qgroup_free_data(inode, page_start, PAGE_SIZE);
+	if (PageDirty(page))
+		btrfs_qgroup_free_data(inode, page_start, PAGE_SIZE);
 	if (!inode_evicting) {
 		clear_extent_bit(tree, page_start, page_end,
 				 EXTENT_LOCKED | EXTENT_DIRTY |
-- 
2.6.6