[Devel] Re: [PATCH v3 02/13] memcg: Reclaim when more than one page needed.

[Johannes Weiner]

On Tue, Sep 18, 2012 at 06:03:59PM +0400, 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

Wow, that patch set has been around for a while.  It's been nr_pages
== 1 for a while now :-)

> 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>
> 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 9d3bc72..b12121b 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2232,7 +2232,8 @@ 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)

I'm not a big fan of the parameter names.  Can we make this function
officially aware of batching and name the parameters like the
arguments that are passed in?  I.e. @batch and @nr_pages?

>  {
>  	unsigned long csize = nr_pages * PAGE_SIZE;
>  	struct mem_cgroup *mem_over_limit;
> @@ -2255,18 +2256,18 @@ 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.

"[...] with the amount of actually required pages instead."

>  	 */
> -	if (nr_pages == CHARGE_BATCH)
> +	if (nr_pages > min_pages)
>  		return CHARGE_RETRY;

	if (batch > nr_pages)
		return CHARGE_RETRY;

But that is all just nitpicking.  Functionally, it looks sane, so:

Acked-by: Johannes Weiner <hannes at cmpxchg.org>