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writeback: keep cleanup_offline_cgwb()'s scan out of syncs

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On a filesystem mounted with lazytime, sync() and syncfs() can return
without persisting timestamp updates of inodes attached to a dying
cgroup wb, so a crash after the sync loses them. File data and sizes are
not affected.

Since commit f6988c9067 ("writeback: bound cleanup_offline_cgwb()
rescans by rotating scanned inodes"), isw_prepare_wbs_switch() moves the
inodes it walks to the newest end of the list. When a pass fills its isw
before reaching an inode that got its lazytime update after a sync's
cutoff, the walked inodes land past it on b_dirty_time, and the sync's
queue_io(), which stops at the first inode dirtied after the cutoff,
misses them. The late inode has to be one without a pending lazytime
update; one already on b_dirty_time keeps its position.

The window is narrow. cleanup_offline_cgwbs_workfn() skips wbs with
anything on b_dirty, b_io or b_more_io, so the scan has to land between
wb_writeback() fixing dirtied_before and taking list_lock for its first
queue_io(), unless a multi-pass drain started before the wb got dirty
IO.

Take wb_switch_rwsem for read across the scan, as
process_inode_switch_wbs() does for the switches.

Fixes: f6988c9067 ("writeback: bound cleanup_offline_cgwb() rescans by rotating scanned inodes")
Cc: stable@vger.kernel.org
Suggested-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/all/0bc5d58533f10b4841807be3025daa11@kernel.org/
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Liz Fong-Jones <lizf@honeycomb.io>
Link: https://patch.msgid.link/20261017040710.1174009-1-lizf@honeycomb.io
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
This commit is contained in:
Liz Fong-Jones 2026-10-17 04:07:11 +00:00 committed by Christian Brauner
parent 8ee09f5977
commit 90c9661246
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GPG Key ID: 91C61BC06578DCA2
1 changed files with 8 additions and 0 deletions
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8
fs/fs-writeback.c
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@ -790,6 +790,13 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb)
new_wb = &wb->bdi->wb; /* wb_get() is noop for bdi's wb */
nr = 0;
/*
* The scan moves walked inodes to the newest end of b_dirty_time,
* past ones it didn't reach. If one of those was dirtied after a
* sync's cutoff, the sync's queue_io() stops there and misses the
* moved ones. Exclude syncs as process_inode_switch_wbs() does.
*/
down_read(&wb->bdi->wb_switch_rwsem);
spin_lock(&wb->list_lock);
/*
* In addition to the inodes that have completed writeback, also switch
@ -804,6 +811,7 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb)
restart = isw_prepare_wbs_switch(new_wb, isw, &wb->b_dirty_time,
&nr);
spin_unlock(&wb->list_lock);
up_read(&wb->bdi->wb_switch_rwsem);
/* no attached inodes? bail out */
if (nr == 0) {