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

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Date:   Wed, 13 Nov 2019 10:26:24 +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

hi Matthew,

Thanks a lot for comments!

在 2019/11/12 下午10:36, Matthew Wilcox 写道:
> On Tue, Nov 12, 2019 at 10:06:24PM +0800, Alex Shi wrote:
>> +/* Don't lock again iff page's lruvec locked */
>> +static inline struct lruvec *relock_page_lruvec_irq(struct page *page,
>> +					struct lruvec *locked_lruvec)
>> +{
>> +	struct pglist_data *pgdat = page_pgdat(page);
>> +	struct lruvec *lruvec;
>> +
>> +	rcu_read_lock();
>> +	lruvec = mem_cgroup_page_lruvec(page, pgdat);
>> +
>> +	if (locked_lruvec == lruvec) {
>> +		rcu_read_unlock();
>> +		return lruvec;
>> +	}
>> +	rcu_read_unlock();
> 
> 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.
 
> 		return lruvec;
> 
> Also, why are you bothering to re-enable interrupts here?  Surely if
> you're holding lock A with interrupts disabled , you can just drop lock A,
> acquire lock B and leave the interrupts alone.  That way you only need
> one of this variety of function, and not the separate irq/irqsave variants.
> 

Thanks for the suggestion! Yes, if only do re-lock, it's better to leave the irq unchanging. but, when the locked_lruvec is NULL, it become a first time lock which irq or irqsave are different. Thus, in combined function we need a nother parameter to indicate if it do irqsaving. So comparing to a extra/indistinct parameter, I guess 2 inline functions would be a bit more simple/cleary? 

Thanks a lot!
Alex