linux-kernel - Re: [v2 PATCH 7/9] mm: vmscan: don't need allocate shrinker->nr

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Message-ID: <20201218010928.GB1199812@dread.disaster.area>
Date:   Fri, 18 Dec 2020 12:09:28 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Yang Shi <shy828301@...il.com>
Cc:     Roman Gushchin <guro@...com>, Kirill Tkhai <ktkhai@...tuozzo.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [v2 PATCH 7/9] mm: vmscan: don't need allocate
 shrinker->nr_deferred for memcg aware shrinkers

On Thu, Dec 17, 2020 at 04:56:48PM -0800, Yang Shi wrote:
> On Tue, Dec 15, 2020 at 3:07 PM Yang Shi <shy828301@...il.com> wrote:
> > > This guarantees that only the shrinker instances taht have a
> > > correctly set up memcg attached to them will have the
> > > SHRINKER_MEMCG_AWARE flag set. Hence in all the rest of the shrinker
> > > code, we only ever need to check for SHRINKER_MEMCG_AWARE to
> > > determine what we should do....
> >
> > Thanks. I see your point. We could move the memcg specific details
> > into prealloc_memcg_shrinker().
> >
> > It seems we have to acquire shrinker_rwsem before we check and modify
> > SHIRNKER_MEMCG_AWARE bit if we may clear it.
> 
> Hi Dave,
> 
> Is it possible that shrinker register races with shrinker unregister?
> It seems impossible to me by a quick visual code inspection. But I'm
> not a VFS expert so I'm not quite sure.

Uh, if you have a shrinker racing to register and unregister, you've
got a major bug in your object initialisation/teardown code. i.e.
calling reagister/unregister at the same time for the same shrinker
is a bug, pure and simple.

Cheers,

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