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FreeBSD-EN-23:18.openzfs                                        Errata Notice
                                                          The FreeBSD Project

Topic:          High CPU usage by ZFS kernel threads

Category:       contrib
Module:         zfs
Announced:      2023-12-05
Affects:        FreeBSD 14.0
Corrected:      2023-11-22 11:43:59 UTC (stable/14, 14.0-STABLE)
                2023-12-05 18:27:35 UTC (releng/14.0, 14.0-RELEASE-p2)

For general information regarding FreeBSD Errata Notices and Security
Advisories, including descriptions of the fields above, security
branches, and the following sections, please visit
<URL:https://security.FreeBSD.org/>.

I.   Background

ZFS is an advanced and scalable file system originally developed by Sun
Microsystems for its Solaris operating system. ZFS was integrated as part of
the FreeBSD starting with FreeBSD 7.0, and it has since become a prominent
and preferred choice for storage management.

II.  Problem Description

Because ZFS may consume large amounts of RAM to cache various types of
filesystem objects, it continuously monitors system RAM available to decide
whether to shrink its caches.  Some caches are shrunk using a dedicated
thread, to which work is dispatched asynchronously.

In some cases, the cache shrinking logic may dispatch excessive amounts of
work to the "ARC pruning" thread, causing it to continue attempting to shrink
caches even after resource shortages are resolved.

III. Impact

The bug manifests as a kernel thread, "arc_prune", consuming 100% of a CPU
core for indefinite periods, even while the system is otherwise idle.  This
behavior may impact workloads running on the system, by reducing available
CPU resources and by triggering lock contention in the kernel.

IV.  Workaround

No workaround is available.  Systems not using ZFS are unaffected.

V.   Solution

Upgrade your system to a supported FreeBSD stable or release / security
branch (releng) dated after the correction date.  A reboot is required
following the upgrade.

Perform one of the following:

1) To update your system via a binary patch:

Systems running a RELEASE version of FreeBSD on the amd64 or arm64 platforms,
or the i386 platfrom on FreeBSD 13 and earlier, can be updated via
the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install

2) To update your system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

# fetch https://security.FreeBSD.org/patches/EN-23:18/openzfs.patch
# fetch https://security.FreeBSD.org/patches/EN-23:18/openzfs.patch.asc
# gpg --verify openzfs.patch.asc

b) Apply the patch.  Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch

c) Recompile your kernel as described in
<URL:https://www.FreeBSD.org/handbook/kernelconfig.html> and reboot the
system.

VI.  Correction details

This issue is corrected as of the corresponding Git commit hash or Subversion
revision number in the following stable and release branches:

Branch/path                             Hash                     Revision
- -------------------------------------------------------------------------
stable/14/                              f7f5c2419ea7    stable/14-n265783
releng/14.0/                            64c5eaab835b  releng/14.0-n265389
- -------------------------------------------------------------------------

Run the following command to see which files were modified by a
particular commit:

# git show --stat <commit hash>

Or visit the following URL, replacing NNNNNN with the hash:

<URL:https://cgit.freebsd.org/src/commit/?id=NNNNNN>

To determine the commit count in a working tree (for comparison against
nNNNNNN in the table above), run:

# git rev-list --count --first-parent HEAD

VII. References

<URL:https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=275063>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-EN-23:18.openzfs.asc>
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