linux-kernel - Re: [PATCH] mm: Optimize put_mems

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Date:	Thu, 17 May 2012 22:23:24 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Mel Gorman <mgorman@...e.de>, Miao Xie <miaox@...fujitsu.com>,
	David Rientjes <rientjes@...gle.com>,
	Christoph Lameter <cl@...ux.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [PATCH] mm: Optimize put_mems_allowed() usage

On Thu, 2012-05-17 at 13:16 -0700, Andrew Morton wrote:
> I do think it was a bad idea to remove that comment.  As it stands, the
> reader will be wondering why we did the read_mems_allowed_begin() at
> all, and whether failing to check for a change is a bug.
> 
> --- a/mm/slub.c~mm-optimize-put_mems_allowed-usage-fix
> +++ a/mm/slub.c
> @@ -1624,8 +1624,16 @@ static struct page *get_any_partial(stru
>                         if (n && cpuset_zone_allowed_hardwall(zone, flags) &&
>                                         n->nr_partial > s->min_partial) {
>                                 object = get_partial_node(s, n, c);
> -                               if (object)
> +                               if (object) {
> +                                       /*
> +                                        * Don't check read_mems_allowed_retry()
> +                                        * here - if mems_allowed was updated in
> +                                        * parallel, that was a harmless race
> +                                        * between allocation and the cpuset
> +                                        * update
> +                                        */
>                                         return object;
> +                               }
>                         }
>                 }
>         } while (read_mems_allowed_retry(cpuset_mems_cookie)); 

OK, it seemed weird to have that comment in this one place whilst it is
the general pattern of this construct.

The whole read_mems_allowed_retry() should only ever be attempted in
case the allocation failed.

But sure..
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