Re: [PATCH v1 1/2] writeback: size foreign flushes by target wb dirty pages

[Tejun Heo <tj@kernel.org>]

On Tue, Sep 08, 2026 at 11:23:51AM +0800, Xin Yin wrote:
> Foreign dirty tracking records the bdi and wb memcg IDs of recently
> dirtied foreign inodes. When the source memcg needs foreign writeback,
> mem_cgroup_flush_foreign() queues WB_REASON_FOREIGN_FLUSH to the
> recorded target bdi_writeback.
> 
> cgroup_writeback_by_id() currently sizes this best-effort work from the
> target memcg's NR_FILE_DIRTY counter. The work is scoped to one target
> wb, so a memcg-wide dirty count can over-size a single-bdi flush when
> the target memcg has dirty pages on other devices. It can also under-size
> the flush when the target wb has dirty pages charged to other memcgs.
> 
> Over-sizing keeps the target wb busy longer and can delay later
> writeback work. Under-sizing can finish before enough pages are written
> back for the source memcg.
> 
> Use the target wb's WB_RECLAIMABLE counter and keep the existing 25%
> headroom. This keeps the budget aligned with the writeback object being
> queued.
> 
> Fixes: 97b27821b485 ("writeback, memcg: Implement foreign dirty flushing")
> Signed-off-by: Xin Yin <yinxin.x@bytedance.com>

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

Thanks.

-- 
tejun