[PATCH v3 1/2] btrfs: fix the delalloc range locking if sector size < page size

[Qu Wenruo <wqu@suse.com>]

Inside lock_delalloc_folios(), there are several problems related to
sector size < page size handling:

- Set the writer locks without checking if the folio is still valid
  We call btrfs_folio_start_writer_lock() just like it's folio_lock().
  But since the folio may not even be the folio of the current mapping,
  we can easily screw up the folio->private.

- The range is not clamped inside the page
  This means we can over write other bitmaps if the start/len is not
  properly handled, and trigger the btrfs_subpage_assert().

- @processed_end is always rounded up to page end
  If the delalloc range is not page aligned, and we need to retry
  (returning -EAGAIN), then we will unlock to the page end.

  Thankfully this is not a huge problem, as now
  btrfs_folio_end_writer_lock() can handle range larger than the locked
  range, and only unlock what is already locked.

Fix all these problems by:

- Lock and check the folio first, then call
  btrfs_folio_set_writer_lock()
  So that if we got a folio not belonging to the inode, we won't
  touch folio->private.

- Properly truncate the range inside the page

- Update @processed_end to the locked range end

Fixes: 1e1de38792e0 ("btrfs: make process_one_page() to handle subpage locking")
Signed-off-by: Qu Wenruo <wqu@suse.com>

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/extent_io.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 9fbc83c76b94..6aa39e0be2e8 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -262,22 +262,23 @@ static noinline int lock_delalloc_folios(struct inode *inode,
 
 		for (i = 0; i < found_folios; i++) {
 			struct folio *folio = fbatch.folios[i];
-			u32 len = end + 1 - start;
+			u64 range_start;
+			u32 range_len;
 
 			if (folio == locked_folio)
 				continue;
 
-			if (btrfs_folio_start_writer_lock(fs_info, folio, start,
-							  len))
-				goto out;
-
+			folio_lock(folio);
 			if (!folio_test_dirty(folio) || folio->mapping != mapping) {
-				btrfs_folio_end_writer_lock(fs_info, folio, start,
-							    len);
+				folio_unlock(folio);
 				goto out;
 			}
+			range_start = max_t(u64, folio_pos(folio), start);
+			range_len = min_t(u64, folio_pos(folio) + folio_size(folio),
+					  end + 1) - range_start;
+			btrfs_folio_set_writer_lock(fs_info, folio, range_start, range_len);
 
-			processed_end = folio_pos(folio) + folio_size(folio) - 1;
+			processed_end = range_start + range_len - 1;
 		}
 		folio_batch_release(&fbatch);
 		cond_resched();
-- 
2.46.2