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

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Message-Id: <20190626205711.379c61b9cdfb9f43ae71c844@linux-foundation.org>
Date:   Wed, 26 Jun 2019 20:57:11 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     zhong jiang <zhongjiang@...wei.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>, <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>
Subject: Re: [PATCH] mm/mempolicy: Fix an incorrect rebind node in
 mpol_rebind_nodemask

On Mon, 27 May 2019 21:58:17 +0800 zhong jiang <zhongjiang@...wei.com> wrote:

> On 2019/5/27 20:23, Vlastimil Babka wrote:
> > On 5/25/19 8:28 PM, Andrew Morton wrote:
> >> (Cc Vlastimil)
> > Oh dear, 2 years and I forgot all the details about how this works.
> >
> >> 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.
> > I have a hard time understanding what the problem is. Could you please
> > write it as a (pseudo) reproducer? I.e. an example of the process/admin
> > mempolicy/cpuset actions that have some wrong observed results vs the
> > correct expected result.
> Sorry, I havn't an testcase to reproduce the issue. At first, It was disappeared by
> my colleague to configure the xml to start an vm.  To his suprise, The bind mempolicy
> doesn't work.

So... what do we do with this patch?

> Thanks,
> zhong jiang
> >>> --- 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;
> > Looks like a mechanical error on my side when removing the code for
> > step1+step2 rebinding. Before my commit there was
> >
> > pol->w.cpuset_mems_allowed = step ? tmp : *nodes;
> >
> > Since 'step' was removed and thus 0, I should have used *nodes indeed.
> > Thanks for catching that.

Was that an ack?

> >>>  	}
> >>>  
> >>>  	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.
> > Yeah the comment was specific for the case that was being removed.
> >
> >> Could someone please propose a comment for the above code block
> >> explaining why we're doing what we do?
> > I'll have to relearn this first...
> >
> >
>