From: Qu Wenruo Date: Tue, 7 Jul 2026 06:54:30 +0000 (+0930) Subject: btrfs: fix a regression where PAGECACHE_TAG_DIRTY is never cleared X-Git-Tag: v7.2-rc5~36^2~2 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=9b73625a4f24971d7a1a07df5d7fd58c07bf3f9f;p=thirdparty%2Flinux.git btrfs: fix a regression where PAGECACHE_TAG_DIRTY is never cleared [BUG] The following script (already submitted as generic/798) will report incorrect dirty page numbers, with 64K page size systems and 4K fs block size: # mkfs.btrfs -s 4k -f $dev # mount $dev $mnt # xfs_io -f -c "pwrite 0 64K" -c fsync -c "cachestat 0 64K" $mnt/foobar Cached: 1, Dirty: 1, Writeback: 0, Evicted: 0, Recently Evicted: 0 Note that the dirtied page number is still 1. [CAUSE] The cachestat() goes through the XArray of the page cache, but instead of checking each folio's flag, it uses the PAGECACHE_TAG_DIRTY tag to report dirty pages. Since commit 095be159f3eb ("btrfs: unify folio dirty flag clearing"), btrfs replaced a folio_clear_dirty_for_io() call inside extent_write_cache_pages() with folio_test_dirty(). This will cause the following call sequence for the folio at file offset 0: extent_write_cache_pages() |- folio_test_dirty() | The folio is still dirty, continue to writeback. | |- extent_writepage() |- extent_writepage_io() |- submit_one_sector() for range [0, 4K) | |- btrfs_folio_clear_dirty() | |- btrfs_folio_set_writeback() | |- folio_start_writeback() | It's the first writeback block, we set the writeback | flag for the folio. | But the folio is still dirty, PAGECACHE_TAG_DIRTY is | kept | |- submit_one_sector() for range [4K, 8K) | |- btrfs_folio_clear_dirty() | |- btrfs_folio_set_writeback() | The folio already has writeback flag, no need to call | folio_start_writeback() | | ... |- submit_one_sector() for range [60K, 64K) |- btrfs_folio_clear_dirty() |- btrfs_folio_set_writeback() The folio already has writeback flag, no need to call folio_start_writeback() So the PAGECACHE_TAG_DIRTY is never cleared. Meanwhile for the old code, before that commit, the sequence looks like: extent_write_cache_pages() |- folio_clear_dirty_for_io() | The folio is still dirty, so continue to writeback. | But the folio dirty flag is cleared now. | |- extent_writepage() |- extent_writepage_io() |- submit_one_sector() for range [0, 4K) | |- btrfs_folio_clear_dirty() | |- btrfs_folio_set_writeback() | |- folio_start_writeback() | |- xas_clear(PAGECACHE_TAG) | | It's the first writeback block, we set the writeback | flag for the folio. | And the folio is not dirty, PAGECACHE_TAG_DIRTY is | cleared | |- submit_one_sector() for range [4K, 8K) | |- btrfs_folio_clear_dirty() | |- btrfs_folio_set_writeback() | The folio already has writeback flag, no need to call | folio_start_writeback() | | ... |- submit_one_sector() for range [60K, 64K) |- btrfs_folio_clear_dirty() |- btrfs_folio_set_writeback() The folio already has writeback flag, no need to call folio_start_writeback() Unlike the new code, old code will clear PAGECACHE_TAG_DIRTY for the first writeback block. There is a deeper problem, dirty and writeback folio flags are updated at very different timing. The dirty flag is only cleared when the last sub-folio block has dirty flag cleared. But the writeback flag is set when the first block starts writeback, and later blocks that go through writeback will not call folio_start_writeback() again. If we rely on folio_start_writeback() to update the PAGECACHE_TAG_DIRTY and PAGECACHE_TAG_TOWRITE, it will always be incorrect in one way or another. [FIX] Do not let folio_start_writeback() do any PAGECACHE_TAG_TOWRITE handling. Instead, manually clear both PAGECACHE_TAG_TOWRITE and PAGECACHE_TAG_DIRTY flags when the folio is no longer dirty during btrfs_subpage_set_writeback(). However this is only a hot-fix, for the long term solution we will follow iomap, by calling folio_start_writeback() immediately for the whole folio, and folio_end_writeback() after all writeback finished for the folio. Fixes: 095be159f3eb ("btrfs: unify folio dirty flag clearing") Reviewed-by: Boris Burkov Signed-off-by: Qu Wenruo Signed-off-by: David Sterba --- diff --git a/fs/btrfs/subpage.c b/fs/btrfs/subpage.c index 56060acac2e9..2a9397be8116 100644 --- a/fs/btrfs/subpage.c +++ b/fs/btrfs/subpage.c @@ -359,6 +359,23 @@ void btrfs_subpage_set_dirty(const struct btrfs_fs_info *fs_info, folio_mark_dirty(folio); } +static void folio_clear_tags(struct folio *folio) +{ + struct address_space *mapping = folio_mapping(folio); + XA_STATE(xas, &mapping->i_pages, folio->index); + unsigned long flags; + + ASSERT(folio_test_locked(folio)); + ASSERT(mapping); + ASSERT(mapping_use_writeback_tags(mapping)); + + xas_lock_irqsave(&xas, flags); + xas_load(&xas); + xas_clear_mark(&xas, PAGECACHE_TAG_DIRTY); + xas_clear_mark(&xas, PAGECACHE_TAG_TOWRITE); + xas_unlock_irqrestore(&xas, flags); +} + /* * Extra clear_and_test function for subpage dirty bitmap. * @@ -403,7 +420,6 @@ void btrfs_subpage_set_writeback(const struct btrfs_fs_info *fs_info, unsigned int start_bit = subpage_calc_start_bit(fs_info, folio, writeback, start, len); unsigned long flags; - bool keep_write; spin_lock_irqsave(&bfs->lock, flags); bitmap_set(bfs->bitmaps, start_bit, len >> fs_info->sectorsize_bits); @@ -413,10 +429,14 @@ void btrfs_subpage_set_writeback(const struct btrfs_fs_info *fs_info, * folio. Doing so can cause WB_SYNC_ALL writepages() to overlook it, * assume writeback is complete, and exit too early — violating sync * ordering guarantees. + * + * Instead we manually clear the DIRTY and TOWRITE tags after the folio + * is no longer dirty. */ - keep_write = folio_test_dirty(folio); if (!folio_test_writeback(folio)) - __folio_start_writeback(folio, keep_write); + __folio_start_writeback(folio, true); + if (!folio_test_dirty(folio)) + folio_clear_tags(folio); spin_unlock_irqrestore(&bfs->lock, flags); }