linux-kernel - Re: [PATCH 10/19] thp: PMD splitting without splitting compound page

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Date:	Wed, 19 Nov 2014 06:57:47 +0000
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
CC:	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>,
	Steve Capper <steve.capper@...aro.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 10/19] thp: PMD splitting without splitting compound page

On Wed, Nov 05, 2014 at 04:49:45PM +0200, Kirill A. Shutemov wrote:
> Current split_huge_page() combines two operations: splitting PMDs into
> tables of PTEs and splitting underlying compound page. This patch
> changes split_huge_pmd() implementation to split the given PMD without
> splitting other PMDs this page mapped with or underlying compound page.
> 
> In order to do this we have to get rid of tail page refcounting, which
> uses _mapcount of tail pages. Tail page refcounting is needed to be able
> to split THP page at any point: we always know which of tail pages is
> pinned (i.e. by get_user_pages()) and can distribute page count
> correctly.
> 
> We can avoid this by allowing split_huge_page() to fail if the compound
> page is pinned. This patch removes all infrastructure for tail page
> refcounting and make split_huge_page() to always return -EBUSY. All
> split_huge_page() users already know how to handle its fail. Proper
> implementation will be added later.
> 
> Without tail page refcounting, implementation of split_huge_pmd() is
> pretty straight-forward.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
...

> diff --git a/arch/powerpc/mm/hugetlbpage.c b/arch/powerpc/mm/hugetlbpage.c
> index 7e70ae968e5f..e4ba17694b6b 100644
> --- a/arch/powerpc/mm/hugetlbpage.c
> +++ b/arch/powerpc/mm/hugetlbpage.c
> @@ -1022,7 +1022,6 @@ int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
>  {
>  	unsigned long mask;
>  	unsigned long pte_end;
> -	struct page *head, *page, *tail;
>  	pte_t pte;
>  	int refs;
>  

This breaks build of powerpc, so you need keep *head and *page as
you do for other architectures.--
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