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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id nk11sm17068952pjb.26.2020.12.21.19.38.58 (version=TLS1 cipher=ECDHE-ECDSA-AES128-SHA bits=128/128); Mon, 21 Dec 2020 19:38:59 -0800 (PST) Date: Mon, 21 Dec 2020 19:38:57 -0800 (PST) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Alex Shi cc: Andrew Morton , Hugh Dickins , Johannes Weiner , Michal Hocko , Vladimir Davydov , Alexander Duyck , Hui Su , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/3] mm/memcg: remove rcu locking for lock_page_lruvec function series In-Reply-To: <1608186532-81218-2-git-send-email-alex.shi@linux.alibaba.com> Message-ID: References: <1608186532-81218-1-git-send-email-alex.shi@linux.alibaba.com> <1608186532-81218-2-git-send-email-alex.shi@linux.alibaba.com> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 17 Dec 2020, Alex Shi wrote: > The rcu_read_lock was used to block memcg destory, but with the detailed > calling conditions, the memcg won't gone since the page is hold. So we > don't need it now, let's remove them to save locking load in debugging. " lock_page_lruvec() and its variants used rcu_read_lock() with the intention of safeguarding against the mem_cgroup being destroyed concurrently; but so long as they are called under the specified conditions (as they are), there is no way for the page's mem_cgroup to be destroyed. Delete the unnecessary rcu_read_lock() and _unlock(). " This has little to do with a "locking load in debugging" - so what? But everything to do with deleting bogosity, the sooner the better. > > Signed-off-by: Alex Shi > Cc: Hugh Dickins Acked-by: Hugh Dickins This really surprised me! Nice change, but how on earth did we not notice until now? The rcu_read_lock() seems to have come in, without explanation, somewhere between lru_lock v9 and v11 (I never saw v10); and I guess I was so used to needing rcu_read_lock() in my own implementation, that I was blind to its irrelevance in yours. Cc'ing Alex Duyck, since he was generally very alert to this kind of thing - be good to have his Ack too. Also Cc'ing Hui Su, who sent a similar but unexplained patch just before yours. > Cc: Johannes Weiner > Cc: Michal Hocko > Cc: Vladimir Davydov > Cc: Andrew Morton > Cc: cgroups@vger.kernel.org > Cc: linux-mm@kvack.org > Cc: linux-kernel@vger.kernel.org > --- > mm/memcontrol.c | 6 ------ > 1 file changed, 6 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index e6b50d068b2f..98bbee1d2faf 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -1356,10 +1356,8 @@ struct lruvec *lock_page_lruvec(struct page *page) > struct lruvec *lruvec; > struct pglist_data *pgdat = page_pgdat(page); > > - rcu_read_lock(); > lruvec = mem_cgroup_page_lruvec(page, pgdat); > spin_lock(&lruvec->lru_lock); > - rcu_read_unlock(); > > lruvec_memcg_debug(lruvec, page); > > @@ -1371,10 +1369,8 @@ struct lruvec *lock_page_lruvec_irq(struct page *page) > struct lruvec *lruvec; > struct pglist_data *pgdat = page_pgdat(page); > > - rcu_read_lock(); > lruvec = mem_cgroup_page_lruvec(page, pgdat); > spin_lock_irq(&lruvec->lru_lock); > - rcu_read_unlock(); > > lruvec_memcg_debug(lruvec, page); > > @@ -1386,10 +1382,8 @@ struct lruvec *lock_page_lruvec_irqsave(struct page *page, unsigned long *flags) > struct lruvec *lruvec; > struct pglist_data *pgdat = page_pgdat(page); > > - rcu_read_lock(); > lruvec = mem_cgroup_page_lruvec(page, pgdat); > spin_lock_irqsave(&lruvec->lru_lock, *flags); > - rcu_read_unlock(); > > lruvec_memcg_debug(lruvec, page); > > -- > 2.29.GIT