linux-kernel - Re: [PATCH] mm: fix account pmd page to the process

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Message-ID: <5764D72B.2080800@huawei.com>
Date:	Sat, 18 Jun 2016 13:07:55 +0800
From:	zhong jiang <zhongjiang@...wei.com>
To:	Mike Kravetz <mike.kravetz@...cle.com>
CC:	"Kirill A. Shutemov" <kirill@...temov.name>,
	Michal Hocko <mhocko@...nel.org>, <akpm@...ux-foundation.org>,
	<linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: fix account pmd page to the process

On 2016/6/17 23:39, Mike Kravetz wrote:
> On 06/17/2016 05:25 AM, Kirill A. Shutemov wrote:
>> From fd22922e7b4664e83653a84331f0a95b985bff0c Mon Sep 17 00:00:00 2001
>> From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
>> Date: Fri, 17 Jun 2016 15:07:03 +0300
>> Subject: [PATCH] hugetlb: fix nr_pmds accounting with shared page tables
>>
>> We account HugeTLB's shared page table to all processes who share it.
>> The accounting happens during huge_pmd_share().
>>
>> If somebody populates pud entry under us, we should decrease pagetable's
>> refcount and decrease nr_pmds of the process.
>>
>> By mistake, I increase nr_pmds again in this case. :-/
>> It will lead to "BUG: non-zero nr_pmds on freeing mm: 2" on process'
>> exit.
>>
>> Let's fix this by increasing nr_pmds only when we're sure that the page
>> table will be used.
>>
>> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Nice,
> Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>
>
> I agree that we do not necessarily need a back port.  I have not seen
> reports of people experiencing this race and seeing the BUG (on mm
> tear-down).
>
> zhongjiang, did someone actually hit the BUG?  Or, did you find it by
> code examination?
>
  just code examination.