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

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Message-ID: <20160617142544.GD6534@node.shutemov.name>
Date:	Fri, 17 Jun 2016 17:25:44 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	Mike Kravetz <mike.kravetz@...cle.com>,
	zhongjiang <zhongjiang@...wei.com>, 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 Fri, Jun 17, 2016 at 03:00:00PM +0200, Michal Hocko wrote:
> On Fri 17-06-16 15:25:06, 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>
> > Reported-by: zhongjiang <zhongjiang@...wei.com>
> > Fixes: dc6c9a35b66b ("mm: account pmd page tables to the process")
> > Cc: <stable@...r.kernel.org>        [4.0+]
> 
> Yes this patch is better. Is it worth backporting to stable though?
> BUG message sounds scary but it is not a real BUG().

I guess, we can live without stable backport.
> 
> Acked-by: Michal Hocko <mhocko@...e.com>

Thanks.

-- 
 Kirill A. Shutemov