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

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Date:	Mon, 3 Jun 2013 15:19:32 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
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 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?

How have you tested this?

> 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.

-- 
Michal Hocko
SUSE Labs
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