Re: [PATCH v2 0/4] btrfs: btrfs: fix possible race between error handling and writeback

David Sterba
2025-08-18T15:48:19Z

On Mon, Jul 28, 2025 at 05:57:53PM +0930, Qu Wenruo wrote:
> [CHANGELOG]
> v2:
> - Add a new patch to explain the error handling better
> This makes the later nocow_one_range() error handling change easier to explain.
>
> There are some rare kernel warning for experimental btrfs builds that
> the DEBUG_WARN() can be triggered from btrfs_writepage_cow_fixup(),
> mostly after some delalloc range failure.
>
> The root cause is explained the the last patch, the TL;DR is we
> shouldn't call btrfs_cleanup_ordered_extents() on folios that are
> already unlocked.
>
> Those unlocked folios can be under writeback, and if we cleared the
> order flag just before the writeback thread entering
> btrfs_writepage_cow_fixup(), we will trigger the warning.
>
> The first patch enhance the error handling of run_delalloc_nocow(), with
> proper comments and charts explaining the cleanup range.
>
> The second patch is a small enhancement to the error messages, which
> helps debugging.
>
> The third patch is to make nocow_one_range() to do proper cleanup,
> aligning itself to cow_file_range().
>
> The last one is to fix the race window by keep folios of successful
> ranges locked, so that we either unlock them manually at the end of
> run_delalloc_nocow(), or get btrfs_cleanup_ordered_extents() called on
> locked folios for error handling.
>
>
> Qu Wenruo (4):
> btrfs: rework the error handling of run_delalloc_nocow()
> btrfs: enhance error messages for delalloc range failure
> btrfs: make nocow_one_range() to do cleanup on error
> btrfs: keep folios locked inside run_delalloc_nocow()

The patches have been in linux-next for some time, feel free to add them
to for-next proper so they can be merged. Related to that, you can
remove the cow fixup code in this development cycle if you want.