[PATCH] btrfs: fix a bug that btrfs_invalidapge() can double account ordered extent for subpage

[Qu Wenruo <wqu@suse.com>]

Commit dbfdb6d1b369 ("Btrfs: Search for all ordered extents that could
span across a page") make btrfs_invalidapage() to search all ordered
extents.

The offending code looks like this:

again:
	start = page_start;
	ordered = btrfs_lookup_ordered_range(inode, start, page_end - start + 1);
	if (ordred) {
		end = min(page_end,
			  ordered->file_offset + ordered->num_bytes - 1);

		/* Do the cleanup */

		start = end + 1;
		if (start < page_end)
			goto again;
	}

The behavior is indeed necessary for the incoming subpage support, but
when it iterate through all the ordered extents, it also resets the
search range @start.

This means, for the following cases, we can double account the ordered
extents, causing its bytes_left underflow:

	Page offset
	0		16K		32K
	|<--- OE 1  --->|<--- OE 2 ---->|

As the first iteration will find OE 1, which doesn't cover the full
page, thus after cleanup code, we need to retry again.
But again label will reset start to page_start, and we got OE 1 again,
which causes double account on OE1, and cause OE1's byte_left to
underflow.

The only good news is, this problem can only happen for subpage case, as
for regular sectorsize == PAGE_SIZE case, we will always find a OE ends
at or after page end, thus no way to trigger the problem.

This patch will move the again label after start = page_start, to fix
the bug.
This is just a quick fix, which is easy to backport.

There will be more comprehensive rework to convert the open coded loop to
a proper while loop.

Fixes: dbfdb6d1b369 ("Btrfs: Search for all ordered extents that could span across a page")
Signed-off-by: Qu Wenruo <wqu@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 ef6cb7b620d0..2eea7d22405a 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -8184,8 +8184,9 @@ static void btrfs_invalidatepage(struct page *page, unsigned int offset,
 
 	if (!inode_evicting)
 		lock_extent_bits(tree, page_start, page_end, &cached_state);
-again:
+
 	start = page_start;
+again:
 	ordered = btrfs_lookup_ordered_range(inode, start, page_end - start + 1);
 	if (ordered) {
 		found_ordered = true;
-- 
2.30.0