Public bug reported:
We observed a kernel panic followed by a ZFS pool hang caused by a race
between dirty mmap page writeback and VFS inode eviction.
The workload is QuestDB, which uses memory-mapped files for its Write-
Ahead Log (WAL). After processing a WAL segment, QuestDB unlinks the
file as part of normal cleanup. In this case, the file was unlinked
while ZFS was still flushing dirty pages belonging to it.
wal-apply_2 was evicting the unlinked inode while the ZFS writeback
thread (flush-zfs-1) was still processing the file. The inode entered
I_FREEING, causing igrab() in zfs_xattr_owner_unlinked() to return NULL
and trigger this assertion:
VERIFY3(igrab(ZTOI((zp))) != NULL)
PANIC at zfs_log.c:257:zfs_xattr_owner_unlinked()
The panic occurred while ZFS had an open DMU transaction, after which
the transaction group could not make progress and the pool hung. The
system required a reboot.
Relevant path:
wb_workfn()
-> zfs_putpage()
-> zfs_log_write()
-> zfs_xattr_owner_unlinked()
-> igrab()
We do not currently have a reliable reproducer.
Expected behaviour
ZFS should handle the inode becoming I_FREEING during this race without
triggering a kernel panic or leaving the ZFS pool unable to make progress.
What happened instead
igrab() returned NULL, the ZFS VERIFY assertion triggered spl_panic(), and the
subsequent failure left the ZFS pool hung until reboot.
#### Environment
Source package: zfs-linux
Ubuntu: 24.04 LTS
Kernel: 6.8.0-138-generic
OpenZFS: 2.2.2-0ubuntu9.4
Architecture: amd64
** Affects: zfs-linux (Ubuntu)
Importance: Undecided
Status: New
** Affects: zfs-linux (Ubuntu Noble)
Importance: Undecided
Status: New
** Also affects: zfs-linux (Ubuntu Noble)
Importance: Undecided
Status: New
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https://bugs.launchpad.net/bugs/2169003
Title:
Panic in zfs_xattr_owner_unlinked() via zhold() on I_FREEING inode
during concurrent mmap writeback
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