Re: [PATCH v3 1/4] mm/zswap: Make shrink_worker writeback cursor per-memcg

[Yosry Ahmed <yosry@kernel.org>]

On Tue, May 26, 2026 at 07:45:58PM +0800, Hao Jia wrote:
> From: Hao Jia <jiahao1@lixiang.com>
> 
> The zswap background writeback worker shrink_worker() uses a global
> cursor zswap_next_shrink, protected by zswap_shrink_lock, to round-robin
> across the online memcgs under root_mem_cgroup.
> 
> Proactive writeback also wants a similar per-memcg cursor that is
> scoped to the specified memcg, so that repeated invocations against
> the same memcg make forward progress across its descendant memcgs
> instead of restarting from the first child memcg each time.

Is this a problem in practice?

Is the concern the overhead of scanning memcgs repeatedly, or lack of
fairness? I wonder if we should just do writeback in batches from all
memcgs, similar to how reclaim does it, then evaluate at the end if we
need to start over?

> 
> Naturally, group the cursor and its protecting spinlock into a
> zswap_wb_iter struct, and make it a member of struct mem_cgroup to
> realize per-memcg cursor management. Accordingly, shrink_worker() now
> uses the lock and cursor in root_mem_cgroup->zswap_wb_iter.

If we really need to have per-memcg cursors (I am not a big fan), I
think we can minimize the overhead by making the cursor updates use
atomic cmpxchg instead of having a per-memcg lock.

> 
> Because the cursor is now per-memcg, the offline cleanup must visit
> every ancestor that could be holding a reference to the dying memcg.
> Factor out __zswap_memcg_offline_cleanup() and walk from dead_memcg up
> to the root.

Another reason why I don't like per-memcg cursors. There is too much
complexity and I wonder if it's warranted. If we stick with per-memcg
cursors please do the refactoring in separate patches to make the
patches easier to review.

Thanks!