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Message-ID: <20090528085503.GB10334@csn.ul.ie>
Date: Thu, 28 May 2009 09:55:04 +0100
From: Mel Gorman <mel@....ul.ie>
To: Ingo Molnar <mingo@...e.hu>
Cc: Andrew Morton <akpm@...ux-foundation.org>, stable@...nel.org,
Linux Memory Management List <linux-mm@...ck.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Hugh Dickins <hugh.dickins@...cali.co.uk>,
Lee Schermerhorn <Lee.Schermerhorn@...com>,
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
starlight@...nacle.cx, Eric B Munson <ebmunson@...ibm.com>,
Adam Litke <agl@...ibm.com>,
Andy Whitcroft <apw@...onical.com>, wli@...ementarian.org
Subject: Re: [PATCH 1/2] x86: Ignore VM_LOCKED when determining if
hugetlb-backed page tables can be shared or not
On Thu, May 28, 2009 at 01:18:03AM +0200, Ingo Molnar wrote:
>
> * Mel Gorman <mel@....ul.ie> wrote:
>
> > On x86 and x86-64, it is possible that page tables are shared
> > beween shared mappings backed by hugetlbfs. As part of this,
> > page_table_shareable() checks a pair of vma->vm_flags and they
> > must match if they are to be shared. All VMA flags are taken into
> > account, including VM_LOCKED.
> >
> > The problem is that VM_LOCKED is cleared on fork(). When a process
> > with a shared memory segment forks() to exec() a helper, there
> > will be shared VMAs with different flags. The impact is that the
> > shared segment is sometimes considered shareable and other times
> > not, depending on what process is checking.
> >
> > What happens is that the segment page tables are being shared but
> > the count is inaccurate depending on the ordering of events. As
> > the page tables are freed with put_page(), bad pmd's are found
> > when some of the children exit. The hugepage counters also get
> > corrupted and the Total and Free count will no longer match even
> > when all the hugepage-backed regions are freed. This requires a
> > reboot of the machine to "fix".
> >
> > This patch addresses the problem by comparing all flags except
> > VM_LOCKED when deciding if pagetables should be shared or not for
> > hugetlbfs-backed mapping.
> >
> > Signed-off-by: Mel Gorman <mel@....ul.ie>
> > Acked-by: Hugh Dickins <hugh.dickins@...cali.co.uk>
> > ---
> > arch/x86/mm/hugetlbpage.c | 6 +++++-
> > 1 files changed, 5 insertions(+), 1 deletions(-)
>
> i suspect it would be best to do this due -mm, due to the (larger)
> mm/hugetlb.c cross section, right?
>
I'm happy with that approach. Almost all hugetlbfs-related patches have
gone through -mm to date AFAIK even when they have been arch specific
like this.
Thanks
--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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