[Devel] Re: [-mm PATCH 6/9] Memory controller add per container LRU and reclaim (v4)

[Vaidyanathan Srinivasan]

Vaidyanathan Srinivasan wrote:
> 
> YAMAMOTO Takashi wrote:
>>> +unsigned long mem_container_isolate_pages(unsigned long nr_to_scan,
>>> +					struct list_head *dst,
>>> +					unsigned long *scanned, int order,
>>> +					int mode, struct zone *z,
>>> +					struct mem_container *mem_cont,
>>> +					int active)
>>> +{
>>> +	unsigned long nr_taken = 0;
>>> +	struct page *page;
>>> +	unsigned long scan;
>>> +	LIST_HEAD(mp_list);
>>> +	struct list_head *src;
>>> +	struct meta_page *mp;
>>> +
>>> +	if (active)
>>> +		src = &mem_cont->active_list;
>>> +	else
>>> +		src = &mem_cont->inactive_list;
>>> +
>>> +	for (scan = 0; scan < nr_to_scan && !list_empty(src); scan++) {
>>> +		mp = list_entry(src->prev, struct meta_page, lru);
>> what prevents another thread from freeing mp here?
> 
> mem_cont->lru_lock protects the list and validity of mp.  If we hold
> mem_cont->lru_lock for this entire loop, then we preserve the validity
> of mp.  However that will be holding up container charge and uncharge.
> 
> This entire routing is called with zone->lru_lock held by the caller.
>  So within a zone, this routine is serialized.
> 
> However page uncharge may race with isolate page.  But will that lead
> to any corruption of the list?  We may be holding the lock for too
> much time just to be on the safe side.
> 
> Please allow us some time to verify whether this is indeed inadequate
> locking that will lead to corruption of the list.

I did few runs and checked for ref_cnt on meta_page and there seems to
be a race between isolate pages and uncharge.  We will probably have
to increase the ref_cnt on meta_page while we are isolating it.  I am
trying to see if we can solve the problem by manipulating the ref_cnt
on the meta_page.

--Vaidy

> Thanks for pointing out this situation.
> --Vaidy
> 
>>> +		spin_lock(&mem_cont->lru_lock);
>>> +		if (mp)
>>> +			page = mp->page;
>>> +		spin_unlock(&mem_cont->lru_lock);
>>> +		if (!mp)
>>> +			continue;
>> YAMAMOTO Takashi
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