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In the iomap_write_iter(), the iomap buffered write frame does not hold
any locks between querying the inode extent mapping info and performing
page cache writes. As a result, the extent mapping can be changed due to
concurrent I/O in flight. Similarly, in the iomap_writepage_map(), the
write-back process faces a similar problem: concurrent changes can
invalidate the extent mapping before the I/O is submitted.
Therefore, both of these processes must recheck the mapping info after
acquiring the folio lock. To address this, similar to XFS, we propose
introducing an extent sequence number to serve as a validity cookie for
the extent. After commit 24b7a2331f ("ext4: clairfy the rules for
modifying extents"), we can ensure the extent information should always
be processed through the extent status tree, and the extent status tree
is always uptodate under i_rwsem or invalidate_lock or folio lock, so
it's safe to introduce this sequence number. The sequence number will be
increased whenever the extent status tree changes, preparing for the
buffered write iomap conversion.
Besides, this mechanism is also applicable for the moving extents case.
In move_extent_per_page(), it also needs to reacquire data_sem and check
the mapping info again under the folio lock.
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Message-ID: <20251013015128.499308-3-yi.zhang@huaweicloud.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
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.