linux-kernel - Re: [PATCHv4 15/24] mm, thp: remove infrastructure for handling splitting PMDs

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Message-ID: <20150329175138.GC976@node.dhcp.inet.fi>
Date:	Sun, 29 Mar 2015 20:51:38 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Hugh Dickins <hughd@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Rik van Riel <riel@...hat.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Christoph Lameter <cl@...two.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Steve Capper <steve.capper@...aro.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Jerome Marchand <jmarchan@...hat.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv4 15/24] mm, thp: remove infrastructure for handling
 splitting PMDs

On Sun, Mar 29, 2015 at 09:40:07PM +0530, Aneesh Kumar K.V wrote:
> "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> writes:
> 
> > With new refcounting we don't need to mark PMDs splitting. Let's drop code
> > to handle this.
> >
> > Arch-specific code will removed separately.
> >
> 
> Can you explain this more ? Why we don't care of PMD splitting case even
> w.r.t to split_huge_page() ? 

It used to be required to keep kernel from updating page refcounts while
we splitting the page. Now with split_huge_pmd() we can split one PMD a
time without blocking refcounting. Once all PMDs split we can freeze the
page's refcounts with compound lock[1] and split underlying compound page.

[1] compound lock is replaced with migration entries by the end of the
    patchset.

-- 
 Kirill A. Shutemov
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