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

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Date:	Mon, 02 Sep 2013 12:37:10 -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

Kirill, thank you for the comment.

On Mon, Sep 02, 2013 at 01:53:27PM +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 which makes us use
> > page->ptl of the pages storing "pmd_trans_huge" pmds.
> 
> Hm. So, you use page->ptl only when you deal with thp pages, otherwise
> mm->page_table_lock, right?

Maybe it's not enough.
We use page->ptl for both of thp and normal depending on USE_SPLIT_PTLOCKS.
And regardless of USE_SPLIT_PTLOCKS, mm->page_table_lock is still used
by other contexts like memory initialization code or driver code for their
specific usage.

> It looks inconsistent to me. Does it mean we have to take both locks on
> split and collapse paths?

This patch includes the replacement with page->ptl for split/collapse path.

> I'm not sure if it's safe to take only
> page->ptl for alloc path. Probably not.

Right, it's not safe.

> Why not to use new locking for pmd everywhere?

So I already do this.

Thanks,
Naoya Horiguchi
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