linux-kernel - Re: Re: [PATCH] futex: bugfix for futex-key conflict when futex use hugepage

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Date:	Fri, 10 May 2013 10:42:22 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	zhang.yi20@....com.cn
Cc:	'Dave Hansen' <dave.hansen@...ux.intel.com>,
	'Darren Hart' <dvhart@...ux.intel.com>,
	linux-kernel@...r.kernel.org, 'Ingo Molnar' <mingo@...nel.org>,
	'Peter Zijlstra' <peterz@...radead.org>,
	'Thomas Gleixner' <tglx@...utronix.de>, wetpzy@....com,
	Zhang Yi <wetpzy@...il.com>
Subject: Re: Re: [PATCH] futex: bugfix for futex-key conflict when futex use
 hugepage

On Fri, May 10, 2013 at 05:08:30PM +0800, zhang.yi20@....com.cn wrote:
> 
> 
> Mel Gorman <mgorman@...e.de> wrote on 2013/05/07 23:20:07:
> 
> >
> > Re: [PATCH] futex: bugfix for futex-key conflict when futex use hugepage
> >
> > On Tue, May 07, 2013 at 08:23:48PM +0800, Zhang Yi wrote:
> > > diff -uprN linux3.9-orig/kernel/futex.c linux3.9/kernel/futex.c
> > > --- linux3.9-orig/kernel/futex.c   2013-04-15 00:45:16.000000000 +0000
> > > +++ linux3.9/kernel/futex.c   2013-05-06 16:24:40.403525000 +0000
> > > @@ -215,6 +215,22 @@ static void drop_futex_key_refs(union fu
> > >     }
> > >  }
> > >
> > > +/*
> > > +* Get subpage index in compound page, and add it into futex_key.
> > > +*/
> > > +static void key_add_compound_idx(union futex_key *key,
> > > +             struct page *head_page, struct page *page)
> > > +{
> > > +   int compound_idx;
> > > +
> > > +   if (compound_order(head_page) >= MAX_ORDER)
> > > +      compound_idx = page_to_pfn(page) - page_to_pfn(head_page);
> > > +   else
> > > +      compound_idx = page - head_page;
> > > +
> > > +   key->both.offset |= compound_idx << PAGE_SHIFT;
> > > +}
> > > +
> >
> > This implicitely assumies it is dealing with a hugetlbfs page. Today, it
> > is the case that an inode-based futex with PageCompound is a hugetlbfs
> > page but that could change in the future if THP ever backs files. This
> > would then break again except it would be harder to fix because THP pages
> > can be collapsed underneath you after the futex key has been generated.
> >
> > As this problem is hugetlbfs-specific should the fix be firmly in
> hugetlbfs
> 
> I think we should do.
> Eg, user applications want high performance, they may use DPDK  which using
> hugetlbfs.
> 
> 
> Should I rework the patch like the following code, and test it?
> 

Yes please, making sure to fix the bug Thomas pointed out.

-- 
Mel Gorman
SUSE Labs
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