linux-kernel - Re: [PATCH 24/29] mm: vmscan: make global slab shrink lockless

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Message-ID: <ZJYaYv4pACmCaBoT@dread.disaster.area>
Date:   Sat, 24 Jun 2023 08:19:14 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Qi Zheng <zhengqi.arch@...edance.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>, paulmck@...nel.org,
        akpm@...ux-foundation.org, tkhai@...ru, roman.gushchin@...ux.dev,
        djwong@...nel.org, brauner@...nel.org, tytso@....edu,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-arm-msm@...r.kernel.org, dm-devel@...hat.com,
        linux-raid@...r.kernel.org, linux-bcache@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-nfs@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-btrfs@...r.kernel.org
Subject: Re: [PATCH 24/29] mm: vmscan: make global slab shrink lockless

On Fri, Jun 23, 2023 at 09:10:57PM +0800, Qi Zheng wrote:
> On 2023/6/23 14:29, Dave Chinner wrote:
> > On Thu, Jun 22, 2023 at 05:12:02PM +0200, Vlastimil Babka wrote:
> > > On 6/22/23 10:53, Qi Zheng wrote:
> > Yes, I suggested the IDR route because radix tree lookups under RCU
> > with reference counted objects are a known safe pattern that we can
> > easily confirm is correct or not.  Hence I suggested the unification
> > + IDR route because it makes the life of reviewers so, so much
> > easier...
> 
> In fact, I originally planned to try the unification + IDR method you
> suggested at the beginning. But in the case of CONFIG_MEMCG disabled,
> the struct mem_cgroup is not even defined, and root_mem_cgroup and
> shrinker_info will not be allocated.  This required more code changes, so
> I ended up keeping the shrinker_list and implementing the above pattern.

Yes. Go back and read what I originally said needed to be done
first. In the case of CONFIG_MEMCG=n, a dummy root memcg still needs
to exist that holds all of the global shrinkers. Then shrink_slab()
is only ever passed a memcg that should be iterated.

Yes, it needs changes external to the shrinker code itself to be
made to work. And even if memcg's are not enabled, we can still use
the memcg structures to ensure a common abstraction is used for the
shrinker tracking infrastructure....

> If the above pattern is not safe, I will go back to the unification +
> IDR method.

And that is exactly how we got into this mess in the first place....

-Dave
-- 
Dave Chinner
david@...morbit.com