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Sun, 07 Mar 2021 22:40:22 -0800 (PST) MIME-Version: 1.0 References: <20210217001322.2226796-1-shy828301@gmail.com> <20210217001322.2226796-4-shy828301@gmail.com> In-Reply-To: <20210217001322.2226796-4-shy828301@gmail.com> From: Shakeel Butt Date: Sun, 7 Mar 2021 22:40:08 -0800 Message-ID: Subject: Re: [v8 PATCH 03/13] mm: vmscan: use shrinker_rwsem to protect shrinker_maps allocation To: Yang Shi Cc: Roman Gushchin , Kirill Tkhai , Vlastimil Babka , Dave Chinner , Johannes Weiner , Michal Hocko , Andrew Morton , Linux MM , linux-fsdevel , LKML Content-Type: text/plain; charset="UTF-8" X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 6CD04132 X-Stat-Signature: 9m935xpgigbaxujb1qgpjrqaiuzgdmmj Received-SPF: none (google.com>: No applicable sender policy available) receiver=imf20; identity=mailfrom; envelope-from=""; helo=mail-lj1-f171.google.com; client-ip=209.85.208.171 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1615185621-714259 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 Tue, Feb 16, 2021 at 4:13 PM Yang Shi wrote: > > Since memcg_shrinker_map_size just can be changed under holding shrinker_rwsem > exclusively, the read side can be protected by holding read lock, so it sounds > superfluous to have a dedicated mutex. > > Kirill Tkhai suggested use write lock since: > > * We want the assignment to shrinker_maps is visible for shrink_slab_memcg(). > * The rcu_dereference_protected() dereferrencing in shrink_slab_memcg(), but > in case of we use READ lock in alloc_shrinker_maps(), the dereferrencing > is not actually protected. > * READ lock makes alloc_shrinker_info() racy against memory allocation fail. > alloc_shrinker_info()->free_shrinker_info() may free memory right after > shrink_slab_memcg() dereferenced it. You may say > shrink_slab_memcg()->mem_cgroup_online() protects us from it? Yes, sure, > but this is not the thing we want to remember in the future, since this > spreads modularity. > > And a test with heavy paging workload didn't show write lock makes things worse. > > Acked-by: Vlastimil Babka > Acked-by: Kirill Tkhai > Acked-by: Roman Gushchin > Signed-off-by: Yang Shi Reviewed-by: Shakeel Butt