linux-kernel - Re: [PATCH] mm/mempolicy: Fix an incorrect rebind node in mpol_rebind

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Message-Id: <20190525112851.ee196bcbbc33bf9e0d869236@linux-foundation.org>
Date:   Sat, 25 May 2019 11:28:51 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     zhong jiang <zhongjiang@...wei.com>
Cc:     <osalvador@...e.de>, <khandual@...ux.vnet.ibm.com>,
        <mhocko@...e.com>, <mgorman@...hsingularity.net>,
        <aarcange@...hat.com>, <rcampbell@...dia.com>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH] mm/mempolicy: Fix an incorrect rebind node in
 mpol_rebind_nodemask

(Cc Vlastimil)

On Sat, 25 May 2019 15:07:23 +0800 zhong jiang <zhongjiang@...wei.com> wrote:

> We bind an different node to different vma, Unluckily,
> it will bind different vma to same node by checking the /proc/pid/numa_maps.   
> Commit 213980c0f23b ("mm, mempolicy: simplify rebinding mempolicies when updating cpusets")
> has introduced the issue.  when we change memory policy by seting cpuset.mems,
> A process will rebind the specified policy more than one times. 
> if the cpuset_mems_allowed is not equal to user specified nodes. hence the issue will trigger.
> Maybe result in the out of memory which allocating memory from same node.
> 
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -345,7 +345,7 @@ static void mpol_rebind_nodemask(struct mempolicy *pol, const nodemask_t *nodes)
>  	else {
>  		nodes_remap(tmp, pol->v.nodes,pol->w.cpuset_mems_allowed,
>  								*nodes);
> -		pol->w.cpuset_mems_allowed = tmp;
> +		pol->w.cpuset_mems_allowed = *nodes;
>  	}
>  
>  	if (nodes_empty(tmp))

hm, I'm not surprised the code broke.  What the heck is going on in
there?  It used to have a perfunctory comment, but Vlastimil deleted
it.

Could someone please propose a comment for the above code block
explaining why we're doing what we do?