[PATCH AUTOSEL 5.1 056/375] Btrfs: fix data bytes_may_use underflow with fallocate due to failed quota reserve

[Sasha Levin <sashal@kernel.org>]

From: Robbie Ko <robbieko@synology.com>

[ Upstream commit 39ad317315887c2cb9a4347a93a8859326ddf136 ]

When doing fallocate, we first add the range to the reserve_list and
then reserve the quota.  If quota reservation fails, we'll release all
reserved parts of reserve_list.

However, cur_offset is not updated to indicate that this range is
already been inserted into the list.  Therefore, the same range is freed
twice.  Once at list_for_each_entry loop, and once at the end of the
function.  This will result in WARN_ON on bytes_may_use when we free the
remaining space.

At the end, under the 'out' label we have a call to:

   btrfs_free_reserved_data_space(inode, data_reserved, alloc_start, alloc_end - cur_offset);

The start offset, third argument, should be cur_offset.

Everything from alloc_start to cur_offset was freed by the
list_for_each_entry_safe_loop.

Fixes: 18513091af94 ("btrfs: update btrfs_space_info's bytes_may_use timely")
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Robbie Ko <robbieko@synology.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/btrfs/file.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 34fe8a58b0e9c..0832449722c12 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -3132,6 +3132,7 @@ static long btrfs_fallocate(struct file *file, int mode,
 			ret = btrfs_qgroup_reserve_data(inode, &data_reserved,
 					cur_offset, last_byte - cur_offset);
 			if (ret < 0) {
+				cur_offset = last_byte;
 				free_extent_map(em);
 				break;
 			}
@@ -3181,7 +3182,7 @@ static long btrfs_fallocate(struct file *file, int mode,
 	/* Let go of our reservation. */
 	if (ret != 0 && !(mode & FALLOC_FL_ZERO_RANGE))
 		btrfs_free_reserved_data_space(inode, data_reserved,
-				alloc_start, alloc_end - cur_offset);
+				cur_offset, alloc_end - cur_offset);
 	extent_changeset_free(data_reserved);
 	return ret;
 }
-- 
2.20.1