linux-kernel - Re: [PATCH 1/2] hugetlbfs: support split page table lock

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Date:	Mon, 03 Jun 2013 10:34:35 -0400
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Andi Kleen <andi@...stfloor.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] hugetlbfs: support split page table lock

On Mon, Jun 03, 2013 at 03:19:32PM +0200, Michal Hocko wrote:
> On Tue 28-05-13 15:52:50, Naoya Horiguchi wrote:
> > Currently all of page table handling by hugetlbfs code are done under
> > mm->page_table_lock. This is not optimal because there can be lock
> > contentions between unrelated components using this lock.
> 
> While I agree with such a change in general I am a bit afraid of all
> subtle tweaks in the mm code that make hugetlb special. Maybe there are
> none for page_table_lock but I am not 100% sure. So this might be
> really tricky and it is not necessary for your further patches, is it?

No, this page_table_lock patch is separable from migration stuff.
As you said in another email, changes going to stable should be minimal,
so it's better to make 2/2 patch not depend on this patch.

> How have you tested this?

Other than libhugetlbfs test (that contains many workloads, but I'm
not sure it can detect the possible regression of this patch,)
I did simple testing where:
 - create a file on hugetlbfs,
 - create 10 processes and make each of them iterate the following:
   * mmap() the hugetlbfs file,
   * memset() the mapped range (to cause hugetlb_fault), and
   * munmap() the mapped range.
I think that this can make racy situation which should be prevented
by page table locks.

> > This patch makes hugepage support split page table lock so that
> > we use page->ptl of the leaf node of page table tree which is pte for
> > normal pages but can be pmd and/or pud for hugepages of some architectures.
> > 
> > Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> > ---
> >  arch/x86/mm/hugetlbpage.c |  6 ++--
> >  include/linux/hugetlb.h   | 18 ++++++++++
> >  mm/hugetlb.c              | 84 ++++++++++++++++++++++++++++-------------------
> 
> This doesn't seem to be the complete story. At least not from the
> trivial:
> $ find arch/ -name "*hugetlb*" | xargs git grep "page_table_lock" -- 
> arch/powerpc/mm/hugetlbpage.c:  spin_lock(&mm->page_table_lock);
> arch/powerpc/mm/hugetlbpage.c:  spin_unlock(&mm->page_table_lock);
> arch/tile/mm/hugetlbpage.c:             spin_lock(&mm->page_table_lock);
> arch/tile/mm/hugetlbpage.c:
> spin_unlock(&mm->page_table_lock);
> arch/x86/mm/hugetlbpage.c: * called with vma->vm_mm->page_table_lock held.

This trivials should be fixed. Sorry.

Naoya
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