[Devel] Re: [PATCH v2 02/11] memcg: Reclaim when more than one page needed.

[Michal Hocko]

On Thu 09-08-12 17:01:10, Glauber Costa wrote:
> From: Suleiman Souhlal <ssouhlal at FreeBSD.org>
> 
> mem_cgroup_do_charge() was written before kmem 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
> both the stack 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) 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 am not happy with the min_pages argument but we can do something more
clever  later.

Acked-by: Michal Hocko <mhocko at suse.cz>

> ---
>  mm/memcontrol.c | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index bc7bfa7..2cef99a 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2294,7 +2294,8 @@ enum {
>  };
>  
>  static int mem_cgroup_do_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
> -				unsigned int nr_pages, bool oom_check)
> +				unsigned int nr_pages, unsigned int min_pages,
> +				bool oom_check)
>  {
>  	unsigned long csize = nr_pages * PAGE_SIZE;
>  	struct mem_cgroup *mem_over_limit;
> @@ -2317,18 +2318,18 @@ 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;
> +
>  	ret = mem_cgroup_reclaim(mem_over_limit, gfp_mask, flags);
>  	if (mem_cgroup_margin(mem_over_limit) >= nr_pages)
>  		return CHARGE_RETRY;
> @@ -2341,7 +2342,7 @@ 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)
>  		return CHARGE_RETRY;
>  
>  	/*
> @@ -2476,7 +2477,8 @@ again:
>  			nr_oom_retries = MEM_CGROUP_RECLAIM_RETRIES;
>  		}
>  
> -		ret = mem_cgroup_do_charge(memcg, gfp_mask, batch, oom_check);
> +		ret = mem_cgroup_do_charge(memcg, gfp_mask, batch, nr_pages,
> +		    oom_check);
>  		switch (ret) {
>  		case CHARGE_OK:
>  			break;
> -- 
> 1.7.11.2
> 
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-- 
Michal Hocko
SUSE Labs