Re: [PATCH] writeback: bound cleanup_offline_cgwb() rescans by rotating b_attached

[Tejun Heo]

On Wed, Sep 09, 2026 at 06:50:27PM +0000, Patrick Lu (Anthropic) wrote:
> cleanup_offline_cgwb() prepares at most WB_MAX_INODES_PER_ISW inodes
> per call and is called again until the dying wb is drained, but every
> call walks wb->b_attached from the head. Inodes already prepared (they
> stay on the list with I_WB_SWITCH set until the switch worker runs) and
> inodes that cannot be switched (I_FREEING, I_WILL_FREE, !SB_ACTIVE,
> DAX, already on the target wb) stay at the head, so each pass rescans a
> growing prefix under wb->list_lock and a full drain is quadratic in the
> number of attached inodes. With ~17M inodes attached to one dying cgwb
> we have seen this end in soft lockups, with CPUs reported stuck for
> 21-48s.
> 
> Move every scanned inode to the tail of b_attached, so the next pass
> starts where the previous one stopped and the drain becomes linear.
> b_attached is unordered and isw_prepare_wbs_switch() is its only
> walker, so nobody else sees the reorder. b_dirty_time is ordered by
> expiry for move_expired_inodes() and keeps its current scan.
> 
> Fixes: c22d70a162d3 ("writeback, cgroup: release dying cgwbs by switching attached inodes")
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: LLM
> Signed-off-by: Patrick Lu (Anthropic) <perf.patrick.lu@xxxxxxxxx>

Acked-by: Tejun Heo <tj@xxxxxxxxxx>

Thanks.

-- 
tejun