Re: [PATCH v2 4/8] mm/lru: only change the lru_lock iff page's lruvec is different

Alex Shi
2019-11-13T02:26:24Z

hi Matthew,

Thanks a lot for comments!

ÔÚ 2019/11/12 ÏÂÎç10:36, Matthew Wilcox Ð´µÀ:
> 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