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[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 commit095be159f3("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:095be159f3("btrfs: unify folio dirty flag clearing") Reviewed-by: Boris Burkov <boris@bur.io> Signed-off-by: Qu Wenruo <wqu@suse.com> Signed-off-by: David Sterba <dsterba@suse.com>
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