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[79.218.188.128]) by smtp.gmail.com with ESMTPSA id z22sm9112468wml.1.2020.12.10.07.36.01 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 10 Dec 2020 07:36:01 -0800 (PST) Date: Thu, 10 Dec 2020 16:33:56 +0100 From: Johannes Weiner To: Yang Shi Cc: guro@fb.com, ktkhai@virtuozzo.com, shakeelb@google.com, david@fromorbit.com, mhocko@suse.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 5/9] mm: memcontrol: add per memcg shrinker nr_deferred Message-ID: <20201210153356.GE264602@cmpxchg.org> References: <20201202182725.265020-1-shy828301@gmail.com> <20201202182725.265020-6-shy828301@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201202182725.265020-6-shy828301@gmail.com> X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Dec 02, 2020 at 10:27:21AM -0800, Yang Shi wrote: > @@ -504,6 +577,34 @@ int memcg_expand_shrinker_maps(int new_id) > return ret; > } > > +int memcg_expand_shrinker_deferred(int new_id) > +{ > + int size, old_size, ret = 0; > + struct mem_cgroup *memcg; > + > + size = (new_id + 1) * sizeof(atomic_long_t); > + old_size = memcg_shrinker_deferred_size; > + if (size <= old_size) > + return 0; > + > + mutex_lock(&memcg_shrinker_mutex); The locking is somewhat confusing. I was wondering why we first read memcg_shrinker_deferred_size "locklessly", then change it while holding the &memcg_shrinker_mutex. memcg_shrinker_deferred_size only changes under shrinker_rwsem(write), correct? This should be documented in a comment, IMO. memcg_shrinker_mutex looks superfluous then. The memcg allocation path is the read-side of memcg_shrinker_deferred_size, and so simply needs to take shrinker_rwsem(read) to lock out shrinker (de)registration. Also, isn't memcg_shrinker_deferred_size just shrinker_nr_max? And memcg_expand_shrinker_deferred() is only called when size >= old_size in the first place (because id >= shrinker_nr_max)?