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

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Message-ID: <1378484835-8552fpnd-mutt-n-horiguchi@ah.jp.nec.com>
Date:	Fri, 06 Sep 2013 12:27:15 -0400
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:	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

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.

> You use uninitialized_var() a lot. It's ugly. I've check few places
> (task_mmu.c, copy_huge_pmd) and have found a reason why we need it there.
> Why?

I got a compile warning of uninitialized usage when developing and added
to suppress it, but in the final form I never get such a warning.
So I'll remove this uninitialized_var()s.

> You often do
> 
> +       ptl = huge_pmd_lockptr(mm, pmd);
> +       spin_lock(ptl);
> 
> Should we have a helper to combine them? huge_pmd_lock()?

OK, I'll do it.

Thanks,
Naoya Horiguchi
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