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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>
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.