Public bug reported:
[Impact]
* Xenial's ZFS can deadlock in the memory shrinker path
after removing files with extended attributes (xattr).
* Extended attributes are enabled by default, but are
_not_ used by default, which reduces the likelyhood.
* It's very difficult/rare to reproduce this problem,
due to file/xattr/remove/shrinker/lru order/timing
circumstances required. (weeks for a reporter user)
but a synthetic test-case has been found for tests.
[Test Case]
* A synthetic reproducer is available for this LP,
with a few steps to touch/setfattr/rm/drop_caches
plus a kernel module to massage the disposal list.
* In the original ZFS module:
the xattr dir inode is not purged immediately on
file removal, but possibly purged _two_ shrinker
invocations later. This allows for other thread
started before file remove to call zfs_zget() on
the xattr child inode and iput() it, so it makes
to the same disposal list as the xattr dir inode.
* In the modified ZFS module:
the xattr dir inode is purged immediately on file
removal not possibly later on shrinker invocation,
so the problem window above doesn't exist anymore.
[Regression Potential]
* Low. The patches are confined to extended attributes
in ZFS, specifically node removal/purge, and another
change how an xattr child inode tracks its xattr dir
(parent) inode, so that it can be purged immediately
on removal.
* The ZFS test-suite has been run on original/modified
zfs-dkms package/kernel modules, with no regressions.
** Affects: zfs-linux (Ubuntu)
Importance: Undecided
Status: Invalid
** Affects: zfs-linux (Ubuntu Xenial)
Importance: Undecided
Assignee: Mauricio Faria de Oliveira (mfo)
Status: In Progress
** Affects: zfs-linux (Ubuntu Bionic)
Importance: Undecided
Status: Invalid
** Affects: zfs-linux (Ubuntu Disco)
Importance: Undecided
Status: Invalid
** Affects: zfs-linux (Ubuntu Eoan)
Importance: Undecided
Status: Invalid
** Also affects: zfs-linux (Ubuntu Bionic)
Importance: Undecided
Status: New
** Also affects: zfs-linux (Ubuntu Xenial)
Importance: Undecided
Status: New
** Also affects: zfs-linux (Ubuntu Eoan)
Importance: Undecided
Status: New
** Also affects: zfs-linux (Ubuntu Disco)
Importance: Undecided
Status: New
** Changed in: zfs-linux (Ubuntu Eoan)
Status: New => Invalid
** Changed in: zfs-linux (Ubuntu Disco)
Status: New => Invalid
** Changed in: zfs-linux (Ubuntu Bionic)
Status: New => Invalid
** Changed in: zfs-linux (Ubuntu Xenial)
Status: New => In Progress
** Changed in: zfs-linux (Ubuntu Xenial)
Assignee: (unassigned) => Mauricio Faria de Oliveira (mfo)
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https://bugs.launchpad.net/bugs/1839521
Title:
Xenial: ZFS deadlock in shrinker path with xattrs
Status in zfs-linux package in Ubuntu:
Invalid
Status in zfs-linux source package in Xenial:
In Progress
Status in zfs-linux source package in Bionic:
Invalid
Status in zfs-linux source package in Disco:
Invalid
Status in zfs-linux source package in Eoan:
Invalid
Bug description:
[Impact]
* Xenial's ZFS can deadlock in the memory shrinker path
after removing files with extended attributes (xattr).
* Extended attributes are enabled by default, but are
_not_ used by default, which reduces the likelyhood.
* It's very difficult/rare to reproduce this problem,
due to file/xattr/remove/shrinker/lru order/timing
circumstances required. (weeks for a reporter user)
but a synthetic test-case has been found for tests.
[Test Case]
* A synthetic reproducer is available for this LP,
with a few steps to touch/setfattr/rm/drop_caches
plus a kernel module to massage the disposal list.
* In the original ZFS module:
the xattr dir inode is not purged immediately on
file removal, but possibly purged _two_ shrinker
invocations later. This allows for other thread
started before file remove to call zfs_zget() on
the xattr child inode and iput() it, so it makes
to the same disposal list as the xattr dir inode.
* In the modified ZFS module:
the xattr dir inode is purged immediately on file
removal not possibly later on shrinker invocation,
so the problem window above doesn't exist anymore.
[Regression Potential]
* Low. The patches are confined to extended attributes
in ZFS, specifically node removal/purge, and another
change how an xattr child inode tracks its xattr dir
(parent) inode, so that it can be purged immediately
on removal.
* The ZFS test-suite has been run on original/modified
zfs-dkms package/kernel modules, with no regressions.
To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/zfs-linux/+bug/1839521/+subscriptions
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