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89cfa89960
fstrim will hold the AGF lock for as long as it takes to walk and discard all the free space in the AG that meets the userspace trim criteria. For AGs with lots of free space extents (e.g. millions) or the underlying device is really slow at processing discard requests (e.g. Ceph RBD), this means the AGF hold time is often measured in minutes to hours, not a few milliseconds as we normal see with non-discard based operations. This can result in the entire filesystem hanging whilst the long-running fstrim is in progress. We can have transactions get stuck waiting for the AGF lock (data or metadata extent allocation and freeing), and then more transactions get stuck waiting on the locks those transactions hold. We can get to the point where fstrim blocks an extent allocation or free operation long enough that it ends up pinning the tail of the log and the log then runs out of space. At this point, every modification in the filesystem gets blocked. This includes read operations, if atime updates need to be made. To fix this problem, we need to be able to discard free space extents safely without holding the AGF lock. Fortunately, we already do this with online discard via busy extents. We can mark free space extents as "busy being discarded" under the AGF lock and then unlock the AGF, knowing that nobody will be able to allocate that free space extent until we remove it from the busy tree. Modify xfs_trim_extents to use the same asynchronous discard mechanism backed by busy extents as is used with online discard. This results in the AGF only needing to be held for short periods of time and it is never held while we issue discards. Hence if discard submission gets throttled because it is slow and/or there are lots of them, we aren't preventing other operations from being performed on AGF while we wait for discards to complete... Signed-off-by: Dave Chinner <dchinner@redhat.com> Reviewed-by: Darrick J. Wong <djwong@kernel.org> |
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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 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.