Re: [PATCH v2] writeback: keep cleanup_offline_cgwb()'s scan out of syncs

[Tejun Heo <tj@kernel.org>]

On Sat, Oct 17, 2026 at 04:07:11AM +0000, Liz Fong-Jones wrote:
> 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 f6988c90671e ("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: f6988c90671e ("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/
> Assisted-by: Claude:claude-opus-5-5 checkpatch
> Assisted-by: Claude:claude-fable-5-1
> Acked-by: Tejun Heo <tj@kernel.org>
> Signed-off-by: Liz Fong-Jones <lizf@honeycomb.io>

Acked-by: Tejun Heo <tj@kernel.org>

Thanks.

--
tejun