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Subject: Re: [PATCH v2 4/8] mm/lru: only change the lru_lock iff page's lruvec
 is different
To:     Matthew Wilcox <willy@infradead.org>
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        linux-mm@kvack.org, akpm@linux-foundation.org,
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References: <1573567588-47048-1-git-send-email-alex.shi@linux.alibaba.com>
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 <20191112143624.GA7934@bombadil.infradead.org>
From:   Alex Shi <alex.shi@linux.alibaba.com>
Message-ID: <297ad71c-081c-f7e1-d640-8720a0eeeeba@linux.alibaba.com>
Date:   Wed, 13 Nov 2019 10:26:24 +0800
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hi Matthew,

Thanks a lot for comments!

?? 2019/11/12 ????10:36, Matthew Wilcox д??:
> 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