[Devel] Re: [PATCH v4 07/25] memcg: Reclaim when more than one page needed.

[Glauber Costa]

On 06/20/2012 05:47 PM, Michal Hocko wrote:
> On Mon 18-06-12 14:28:00, Glauber Costa wrote:
>> From: Suleiman Souhlal <ssouhlal at FreeBSD.org>
>>
>> mem_cgroup_do_charge() was written before slab accounting, and expects
>> three cases: being called for 1 page, being called for a stock of 32 pages,
>> or being called for a hugepage.  If we call for 2 or 3 pages (and several
>> slabs used in process creation are such, at least with the debug options I
>> had), it assumed it's being called for stock and just retried without reclaiming.
>>
>> Fix that by passing down a minsize argument in addition to the csize.
>>
>> And what to do about that (csize == PAGE_SIZE && ret) retry?  If it's
>> needed at all (and presumably is since it's there, perhaps to handle
>> races), then it should be extended to more than PAGE_SIZE, yet how far?
>> And should there be a retry count limit, of what?  For now retry up to
>> COSTLY_ORDER (as page_alloc.c does), stay safe with a cond_resched(),
>> and make sure not to do it if __GFP_NORETRY.
>>
>> [v4: fixed nr pages calculation pointed out by Christoph Lameter ]
>>
>> Signed-off-by: Suleiman Souhlal <suleiman at google.com>
>> Signed-off-by: Glauber Costa <glommer at parallels.com>
>> Reviewed-by: Kamezawa Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
>
> I think this is not ready to be merged yet.
Fair Enough

> Two comments below.
>
> [...]
>> @@ -2210,18 +2211,18 @@ static int mem_cgroup_do_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
>>   	} else
>>   		mem_over_limit = mem_cgroup_from_res_counter(fail_res, res);
>>   	/*
>> -	 * nr_pages can be either a huge page (HPAGE_PMD_NR), a batch
>> -	 * of regular pages (CHARGE_BATCH), or a single regular page (1).
>> -	 *
>>   	 * Never reclaim on behalf of optional batching, retry with a
>>   	 * single page instead.
>>   	 */
>> -	if (nr_pages == CHARGE_BATCH)
>> +	if (nr_pages > min_pages)
>>   		return CHARGE_RETRY;
>>
>>   	if (!(gfp_mask & __GFP_WAIT))
>>   		return CHARGE_WOULDBLOCK;
>>
>> +	if (gfp_mask & __GFP_NORETRY)
>> +		return CHARGE_NOMEM;
>
> This is kmem specific and should be preparated out in case this should
> be merged before the rest.
ok.

> Btw. I assume that oom==false when called from kmem...

What prevents the oom killer to be called for a reclaimable kmem 
allocation that can be satisfied ?

>> +
>>   	ret = mem_cgroup_reclaim(mem_over_limit, gfp_mask, flags);
>>   	if (mem_cgroup_margin(mem_over_limit) >= nr_pages)
>>   		return CHARGE_RETRY;
>> @@ -2234,8 +2235,10 @@ static int mem_cgroup_do_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
>>   	 * unlikely to succeed so close to the limit, and we fall back
>>   	 * to regular pages anyway in case of failure.
>>   	 */
>> -	if (nr_pages == 1 && ret)
>> +	if (nr_pages <= (1 << PAGE_ALLOC_COSTLY_ORDER) && ret) {
>> +		cond_resched();
>>   		return CHARGE_RETRY;
>> +	}
>
> What prevents us from looping for unbounded amount of time here?
> Maybe you need to consider the number of reclaimed pages here.

Why would we even loop here? It will just return CHARGE_RETRY, it is up 
to the caller to decide whether or not it will retry.