Re: [PATCH v3 3/7] mm/lru: replace pgdat lru_lock with lruvec lock

[Alex Shi]

在 2019/11/16 下午12:38, Matthew Wilcox 写道:
> On Sat, Nov 16, 2019 at 11:15:02AM +0800, Alex Shi wrote:
>> This is the main patch to replace per node lru_lock with per memcg
>> lruvec lock. It also fold the irqsave flags into lruvec.
> 
> I have to say, I don't love the part where we fold the irqsave flags
> into the lruvec.  I know it saves us an argument, but it opens up the
> possibility of mismatched expectations.  eg we currently have:
> 
> static void __split_huge_page(struct page *page, struct list_head *list,
> 			struct lruvec *lruvec, pgoff_t end)
> {
> ...
> 	spin_unlock_irqrestore(&lruvec->lru_lock, lruvec->irqflags);
> 
> so if we introduce a new caller, we have to be certain that this caller
> is also using lock_page_lruvec_irqsave() and not lock_page_lruvec_irq().
> I can't think of a way to make the compiler enforce that, and if we don't,
> then we can get some odd crashes with interrupts being unexpectedly
> enabled or disabled, depending on how ->irqflags was used last.
> 
> So it makes the code more subtle.  And that's not a good thing.

Hi Matthew,

Thanks for comments!

Here, the irqflags is bound, and belong to lruvec, merging them into together helps us to take them as whole, and thus reduce a unnecessary code clues.
The only thing maybe bad that it may take move place in pg_data_t.lruvec, but there are PADDINGs to remove this concern.

As your concern for a 'new' caller, since __split_huge_page is a static helper here, no distub for anyothers.

Do you agree on that?

> 
>> +static inline struct lruvec *lock_page_lruvec_irq(struct page *page,
>> +						struct pglist_data *pgdat)
>> +{
>> +	struct lruvec *lruvec = mem_cgroup_page_lruvec(page, pgdat);
>> +
>> +	spin_lock_irq(&lruvec->lru_lock);
>> +
>> +	return lruvec;
>> +}
> 
> ...
> 
>> +static struct lruvec *lock_page_lru(struct page *page, int *isolated)
>>  {
>>  	pg_data_t *pgdat = page_pgdat(page);
>> +	struct lruvec *lruvec = lock_page_lruvec_irq(page, pgdat);
>>  
>> -	spin_lock_irq(&pgdat->lru_lock);
>>  	if (PageLRU(page)) {
>> -		struct lruvec *lruvec;
>>  
>> -		lruvec = mem_cgroup_page_lruvec(page, pgdat);
>>  		ClearPageLRU(page);
>>  		del_page_from_lru_list(page, lruvec, page_lru(page));
>>  		*isolated = 1;
>>  	} else
>>  		*isolated = 0;
>> +
>> +	return lruvec;
>>  }
> 
> But what if the page is !PageLRU?  What lruvec did we just lock?

like original pgdat->lru_lock, we need the lock from PageLRU racing. And it the lruvec which the page should be.


> According to the comments on mem_cgroup_page_lruvec(),
> 
>  * This function is only safe when following the LRU page isolation
>  * and putback protocol: the LRU lock must be held, and the page must
>  * either be PageLRU() or the caller must have isolated/allocated it.
> 
> and now it's being called in order to find out which LRU lock to take.
> So this comment needs to be updated, if it's wrong, or this patch has
> a race.


Yes, the function reminder is a bit misunderstanding with new patch, How about the following changes:

- * This function is only safe when following the LRU page isolation
- * and putback protocol: the LRU lock must be held, and the page must
- * either be PageLRU() or the caller must have isolated/allocated it.
+ * The caller needs to grantee the page's mem_cgroup is undisturbed during
+ * using. That could be done by lock_page_memcg or lock_page_lruvec.

Thanks
Alex