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Date: Tue, 3 Jul 2018 21:39:02 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: [PATCH v8 03/17] mm: Assign id to every memcg-aware shrinker
Message-ID: <20180703203901.GV30522@ZenIV.linux.org.uk>
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To: Matthew Wilcox <willy@infradead.org>
Cc: Shakeel Butt <shakeelb@google.com>, Kirill Tkhai <ktkhai@virtuozzo.com>, Vladimir Davydov <vdavydov.dev@gmail.com>, Johannes Weiner <hannes@cmpxchg.org>, Michal Hocko <mhocko@kernel.org>, Thomas Gleixner <tglx@linutronix.de>, Philippe Ombredanne <pombredanne@nexb.com>, stummala@codeaurora.org, gregkh@linuxfoundation.org, Stephen Rothwell <sfr@canb.auug.org.au>, Roman Gushchin <guro@fb.com>, mka@chromium.org, Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>, Chris Wilson <chris@chris-wilson.co.uk>, longman@redhat.com, Minchan Kim <minchan@kernel.org>, Huang Ying <ying.huang@intel.com>, Mel Gorman <mgorman@techsingularity.net>, jbacik@fb.com, Guenter Roeck <linux@roeck-us.net>, LKML <linux-kernel@vger.kernel.org>, Linux MM <linux-mm@kvack.org>, lirongqing@baidu.com, Andrey Ryabinin <aryabinin@virtuozzo.com>, Andrew Morton <akpm@linux-foundation.org>

On Tue, Jul 03, 2018 at 10:47:44AM -0700, Matthew Wilcox wrote:
> On Tue, Jul 03, 2018 at 08:46:28AM -0700, Shakeel Butt wrote:
> > On Tue, Jul 3, 2018 at 8:27 AM Matthew Wilcox <willy@infradead.org> wrote:
> > > This will actually reduce the size of each shrinker and be more
> > > cache-efficient when calling the shrinkers.  I think we can also get
> > > rid of the shrinker_rwsem eventually, but let's leave it for now.
> > 
> > Can you explain how you envision shrinker_rwsem can be removed? I am
> > very much interested in doing that.
> 
> Sure.  Right now we have 3 uses of shrinker_rwsem -- two for adding and
> removing shrinkers to the list and one for walking the list.  If we switch
> to an IDR then we can use a spinlock for adding/removing shrinkers and
> the RCU read lock for looking up an entry in the IDR each iteration of
> the loop.
> 
> We'd need to stop the shrinker from disappearing underneath us while we
> drop the RCU lock, so we'd need a refcount in the shrinker, and to free
> the shrinkers using RCU.  We do similar things for other data structures,
> so this is all pretty well understood.

<censored>
struct super_block {
...
        struct shrinker s_shrink;       /* per-sb shrinker handle */
...
}
<censored>

What was that about refcount in the shrinker and taking over the lifetime
rules of the objects it might be embedded into, again?