Re: [PATCH v4 3/9] mm/lru: replace pgdat lru_lock with lruvec lock

[Alex Shi]

在 2019/11/20 上午12:04, Johannes Weiner 写道:
>> +
>> +	return lruvec;
> While this works in practice, it looks wrong because it doesn't follow
> the mem_cgroup_page_lruvec() rules.
> 
> Please open-code spin_lock_irq(&pgdat->__lruvec->lru_lock) instead.
> 

That's right. Thanks for suggestion!

>> @@ -1246,6 +1245,46 @@ struct lruvec *mem_cgroup_page_lruvec(struct page *page, struct pglist_data *pgd
>>  	return lruvec;
>>  }
>>  
>> +struct lruvec *lock_page_lruvec_irq(struct page *page,
>> +					struct pglist_data *pgdat)
>> +{
>> +	struct lruvec *lruvec;
>> +
>> +again:
>> +	rcu_read_lock();
>> +	lruvec = mem_cgroup_page_lruvec(page, pgdat);
>> +	spin_lock_irq(&lruvec->lru_lock);
>> +	rcu_read_unlock();
> The spinlock doesn't prevent the lruvec from being freed
> 
> You deleted the rules from the mem_cgroup_page_lruvec() documentation,
> but they still apply: if the page is already !PageLRU() by the time
> you get here, it could get reclaimed or migrated to another cgroup,
> and that can free the memcg/lruvec. Merely having the lru_lock held
> does not prevent this.


Forgive my idiot, I still don't know the details of unsafe lruvec here.
>From my shortsight, the spin_lock_irq(embedded a preempt_disable) could block all rcu syncing thus, keep all memcg alive until the preempt_enabled in unspinlock, is this right?
If so even the page->mem_cgroup is migrated to others cgroups, the new and old cgroup should still be alive here. 

> 
> Either the page needs to be locked, or the page needs to be PageLRU
> with the lru_lock held to prevent somebody else from isolating
> it. Otherwise, the lruvec is not safe to use.

Do you mean that we may get the wrong lruvec->lru_lock if !PageLRU, so, the page may got freed by others? Sorry I got last there.

Thanks
Alex