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

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Message-Id: <20130906164625.353B1E0090@blue.fi.intel.com>
Date:	Fri,  6 Sep 2013 19:46:25 +0300 (EEST)
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Andi Kleen <andi@...stfloor.org>,
	Michal Hocko <mhocko@...e.cz>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Alex Thorlton <athorlton@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] thp: support split page table lock

Naoya Horiguchi wrote:
> Hi Kirill,
> 
> On Fri, Sep 06, 2013 at 01:48:03PM +0300, Kirill A. Shutemov wrote:
> > Naoya Horiguchi wrote:
> > > Thp related code also uses per process mm->page_table_lock now.
> > > So making it fine-grained can provide better performance.
> > > 
> > > This patch makes thp support split page table lock by using page->ptl
> > > of the pages storing "pmd_trans_huge" pmds.
> > > 
> > > Some functions like pmd_trans_huge_lock() and page_check_address_pmd()
> > > are expected by their caller to pass back the pointer of ptl, so this
> > > patch adds to those functions new arguments for that. Rather than that,
> > > this patch gives only straightforward replacement.
> > > 
> > > ChangeLog v3:
> > >  - fixed argument of huge_pmd_lockptr() in copy_huge_pmd()
> > >  - added missing declaration of ptl in do_huge_pmd_anonymous_page()
> > > 
> > > Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> > 
> > Generally, looks good. Few notes:
> > 
> > I believe you need to convert __pte_alloc() to new locking. Not sure about
> > __pte_alloc_kernel().
> > Have you check all rest mm->page_table_lock, that they shouldn't be
> > converted to new locking?
> 
> I thought that keeping __pte_alloc() using mm->page_table_lock was safe
> because it uses bare mm->page_table_lock instead of pte_lockptr() even
> before this patchset, but not 100% sure.
> __pte_alloc() (and its family) are used in normal page path, so if it's
> not safe, we've lived with unsafe code for very long (maybe since 2005).
> Anyway, converting __pte_alloc() into split ptl could improve performance
> (though we need testing to know what amount), so I'll try that.

No, before the patch mm->page_table_lock is what we need: it serializes
setting up pmd, not adding pte to pmd and it's subject to change with new
locking model.

-- 
 Kirill A. Shutemov
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