linux-kernel - Re: [PATCHv4 12/24] thp: PMD splitting without splitting compound page

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Date:	Tue, 17 Mar 2015 14:02:36 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>
Cc:	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,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCHv4 12/24] thp: PMD splitting without splitting compound page

"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> writes:

> 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.
>

mm/gup.c: In function ‘gup_huge_pgd’:
mm/gup.c:1183:2: error: ‘tail’ undeclared (first use in this function)
  tail = page;
  ^
diff --git a/mm/gup.c b/mm/gup.c
index 2f776850108c..141b5a81cf8a 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1180,7 +1180,6 @@ static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
 	refs = 0;
 	head = pgd_page(orig);
 	page = head + ((addr & ~PGDIR_MASK) >> PAGE_SHIFT);
-	tail = page;
 	do {
 		VM_BUG_ON_PAGE(compound_head(page) != head, page);
 		pages[*nr] = page;

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