Re: [PATCH/RFC] btrfs: fix folio lock leak in writepage_delalloc() for folios dirtied behind btrfs' back

[Matthew Wilcox]

On Wed, Jul 22, 2026 at 11:29:36AM +0200, Christian Borntraeger wrote:
>  *   4. Thread B loops sync_file_range(WRITE|WAIT) on the target file.
>  *      Whenever a full clean cycle (clear_page_dirty_for_io(),
>  *      writeback, bits cleared) completes inside thread A's
>  *      submission->completion window, the completion-time
>  *      set_page_dirty_lock() hits a *clean* folio: filemap_dirty_folio()
>  *      sets only the folio flag and the xarray tag - no btrfs subpage
>  *      dirty bit, no delalloc reservation.  See the 20-year-old comment
>  *      above bio_set_pages_dirty() in block/bio.c describing exactly
>  *      this ("other code (eg, flusher threads) could clean the pages").

There's your problem.  filemap_dirty_folio() documents that btrfs is
doing it wrongly:

 * Filesystems which do not use buffer heads should call this function
 * from their dirty_folio address space operation.  It ignores the
 * contents of folio_get_private(), so if the filesystem marks individual
 * blocks as dirty, the filesystem should handle that itself.

fs/btrfs/inode.c:       .dirty_folio    = filemap_dirty_folio,

so btrfs should have its own btrfs_dirty_folio() which does whatever
metadata updates it needs to and then call filemap_dirty_folio() to
take care of the page cache business.  See iomap_dirty_folio() as
an example, but many other filesystems also do this.