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

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Date:   Thu, 27 Jun 2019 09:47:41 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        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
Subject: Re: [PATCH] mm/mempolicy: Fix an incorrect rebind node in
 mpol_rebind_nodemask

On 6/27/19 5:57 AM, Andrew Morton wrote:
> 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?

The fix should be fine, but the description is vague. I'll try to
improve it.

>>>>>  	}
>>>>>  
>>>>>  	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...
>>>
>>>
>>