btrfs: mark all dirty sectors as locked inside writepage_delalloc()

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[ Upstream commit c96d0e3921 ]

Currently we only mark sectors as locked if there is a *NEW* delalloc
range for it.

But NEW delalloc range is not the same as dirty sectors we want to
submit, e.g:

        0       32K      64K      96K       128K
        |       |////////||///////|    |////|
                                       120K

For above 64K page size case, writepage_delalloc() for page 0 will find
and lock the delalloc range [32K, 96K), which is beyond the page
boundary.

Then when writepage_delalloc() is called for the page 64K, since [64K,
96K) is already locked, only [120K, 128K) will be locked.

This means, although range [64K, 96K) is dirty and will be submitted
later by extent_writepage_io(), it will not be marked as locked.

This is fine for now, as we call btrfs_folio_end_writer_lock_bitmap() to
free every non-compressed sector, and compression is only allowed for
full page range.

But this is not safe for future sector perfect compression support, as
this can lead to double folio unlock:

              Thread A                 |           Thread B
---------------------------------------+--------------------------------
                                       | submit_one_async_extent()
				       | |- extent_clear_unlock_delalloc()
extent_writepage()                     |    |- btrfs_folio_end_writer_lock()
|- btrfs_folio_end_writer_lock_bitmap()|       |- btrfs_subpage_end_and_test_writer()
   |                                   |       |  |- atomic_sub_and_test()
   |                                   |       |     /* Now the atomic value is 0 */
   |- if (atomic_read() == 0)          |       |
   |- folio_unlock()                   |       |- folio_unlock()

The root cause is the above range [64K, 96K) is dirtied and should also
be locked but it isn't.

So to make everything more consistent and prepare for the incoming
sector perfect compression, mark all dirty sectors as locked.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Stable-dep-of: 8bf334beb3 ("btrfs: fix double accounting race when extent_writepage_io() failed")
Signed-off-by: Sasha Levin <sashal@kernel.org>

...

This commit is contained in:

Qu Wenruo 2024-09-16 08:12:40 +09:30 • committed by Greg Kroah-Hartman

parent 4dd35a361f

commit 9f9e6f07cd

1 changed files with 7 additions and 3 deletions

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fs/btrfs/extent_io.c

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