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Date: Mon, 21 Dec 2020 19:38:57 -0800 (PST) From: Hugh Dickins <hughd@...gle.com> To: Alex Shi <alex.shi@...ux.alibaba.com> cc: Andrew Morton <akpm@...ux-foundation.org>, Hugh Dickins <hughd@...gle.com>, Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, Vladimir Davydov <vdavydov.dev@...il.com>, Alexander Duyck <alexander.duyck@...il.com>, Hui Su <sh_def@....com>, cgroups@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH 2/3] mm/memcg: remove rcu locking for lock_page_lruvec function series 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 <alex.shi@...ux.alibaba.com> > Cc: Hugh Dickins <hughd@...gle.com> Acked-by: Hugh Dickins <hughd@...gle.com> 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 <hannes@...xchg.org> > Cc: Michal Hocko <mhocko@...nel.org> > Cc: Vladimir Davydov <vdavydov.dev@...il.com> > Cc: Andrew Morton <akpm@...ux-foundation.org> > Cc: cgroups@...r.kernel.org > Cc: linux-mm@...ck.org > Cc: linux-kernel@...r.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
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