linux-kernel - Re: [PATCH v1 7/9] workingset: memcg: sleep when flushing stats in workingset

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Date:   Tue, 28 Mar 2023 14:47:46 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Yosry Ahmed <yosryahmed@...gle.com>, Tejun Heo <tj@...nel.org>,
        Josef Bacik <josef@...icpanda.com>,
        Jens Axboe <axboe@...nel.dk>,
        Zefan Li <lizefan.x@...edance.com>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Koutný <mkoutny@...e.com>,
        Vasily Averin <vasily.averin@...ux.dev>,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH v1 7/9] workingset: memcg: sleep when flushing stats in
 workingset_refault()

On Tue, Mar 28, 2023 at 08:18:11AM -0700, Shakeel Butt wrote:
> > @@ -406,6 +406,8 @@ void workingset_refault(struct folio *folio, void *shadow)
> >         unpack_shadow(shadow, &memcgid, &pgdat, &eviction, &workingset);
> >         eviction <<= bucket_order;
> >
> > +       /* Flush stats (and potentially sleep) before holding RCU read lock */
> 
> I think the only reason we use rcu lock is due to
> mem_cgroup_from_id(). Maybe we should add mem_cgroup_tryget_from_id().
> The other caller of mem_cgroup_from_id() in vmscan is already doing
> the same and could use mem_cgroup_tryget_from_id().

Good catch. Nothing else in there is protected by RCU. We can just
hold the ref instead.

> Though this can be done separately to this series (if we decide to do
> it at all).

Agreed