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commit 14a537983b228cb050ceca3a5b743d01315dc4aa upstream. One of our VM cluster management products needs to snapshot KVM image files so that they can be restored in case of failure. Snapshotting is done by redirecting VM disk writes to a sidecar file and using reflink on the disk image, specifically the FICLONE ioctl as used by "cp --reflink". Reflink locks the source and destination files while it operates, which means that reads from the main vm disk image are blocked, causing the vm to stall. When an image file is heavily fragmented, the copy process could take several minutes. Some of the vm image files have 50-100 million extent records, and duplicating that much metadata locks the file for 30 minutes or more. Having activities suspended for such a long time in a cluster node could result in node eviction. Clone operations and read IO do not change any data in the source file, so they should be able to run concurrently. Demote the exclusive locks taken by FICLONE to shared locks to allow reads while cloning. While a clone is in progress, writes will take the IOLOCK_EXCL, so they block until the clone completes. Link: https://lore.kernel.org/linux-xfs/8911B94D-DD29-4D6E-B5BC-32EAF1866245@oracle.com/ Signed-off-by: Catherine Hoang <catherine.hoang@oracle.com> Reviewed-by: "Darrick J. Wong" <djwong@kernel.org> Reviewed-by: Dave Chinner <dchinner@redhat.com> Reviewed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Chandan Babu R <chandanbabu@kernel.org> Acked-by: Chandan Babu R <chandanbabu@kernel.org> Signed-off-by: Sasha Levin <sashal@kernel.org>
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 Restructured Text 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.