linux-kernel - Re: [PATCH v2 4/8] mm/lru: only change the lru

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Message-ID: <ba11d04a-def4-10b3-0b4e-4194e0ee2318@linux.alibaba.com>
Date:   Thu, 14 Nov 2019 14:01:06 +0800
From:   Alex Shi <alex.shi@...ux.alibaba.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, akpm@...ux-foundation.org,
        mgorman@...hsingularity.net, tj@...nel.org, hughd@...gle.com,
        khlebnikov@...dex-team.ru, daniel.m.jordan@...cle.com,
        yang.shi@...ux.alibaba.com, Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Roman Gushchin <guro@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Chris Down <chris@...isdown.name>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        swkhack <swkhack@...il.com>,
        "Potyra, Stefan" <Stefan.Potyra@...ktrobit.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Peng Fan <peng.fan@....com>,
        Nikolay Borisov <nborisov@...e.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Yafang Shao <laoar.shao@...il.com>
Subject: Re: [PATCH v2 4/8] mm/lru: only change the lru_lock iff page's lruvec
 is different



在 2019/11/13 下午9:45, Matthew Wilcox 写道:
>>> Why not simply:
>>>
>>> 	rcu_read_lock();
>>> 	lruvec = mem_cgroup_page_lruvec(page, pgdat);
>>> 	rcu_read_unlock();
>>>
>>> 	if (locked_lruvec == lruvec)
>> The rcu_read_unlock here is for guarding the locked_lruvec/lruvec comparsion.
>> Otherwise memcg/lruvec maybe changed, like, from memcg migration etc. I guess.
> How does holding the RCU lock guard the comparison?  You're comparing two
> pointers for equality.  Nothing any other CPU can do at this point will
> change the results of that comparison.
> 

Hi Matthew, Thanks for reply!

The reason of guarding lruvec compasion here is a bit undistinct, in fact, it guards the page's memcg from free/migrate etc, since the lruvec is kind of binding to each of memcg. The detailed explanation could be found in lock_page_memcg().

Thanks
Alex