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SubjectRe: [PATCH v4 07/25] memcg: Reclaim when more than one page needed.
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@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@google.com>
>> Signed-off-by: Glauber Costa <glommer@parallels.com>
>> Reviewed-by: Kamezawa Hiroyuki <kamezawa.hiroyu@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.


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