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

Qu Wenruo
2025-07-28T08:28:17Z

[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()

fs/btrfs/inode.c | 259 +++++++++++++++++++++++------------------------
1 file changed, 128 insertions(+), 131 deletions(-)

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2.50.1