linux-kernel - [PATCHv10 06/36] mm: handle PTE-mapped tail pages in gerneric fast gup implementaiton

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Date:	Thu,  3 Sep 2015 18:12:52 +0300
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>
Cc:	Dave Hansen <dave.hansen@...el.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>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Jerome Marchand <jmarchan@...hat.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: [PATCHv10 06/36] mm: handle PTE-mapped tail pages in gerneric fast gup implementaiton

With new refcounting we are going to see THP tail pages mapped with PTE.
Generic fast GUP rely on page_cache_get_speculative() to obtain
reference on page. page_cache_get_speculative() always fails on tail
pages, because ->_count on tail pages is always zero.

Let's handle tail pages in gup_pte_range().

New split_huge_page() will rely on migration entries to freeze page's
counts. Recheck PTE value after page_cache_get_speculative() on head
page should be enough to serialize against split.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
Tested-by: Sasha Levin <sasha.levin@...cle.com>
Tested-by: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
Acked-by: Jerome Marchand <jmarchan@...hat.com>
Acked-by: Vlastimil Babka <vbabka@...e.cz>
---
 mm/gup.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 745a50f2d57d..6880085d3790 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1097,7 +1097,7 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
 		 * for an example see gup_get_pte in arch/x86/mm/gup.c
 		 */
 		pte_t pte = READ_ONCE(*ptep);
-		struct page *page;
+		struct page *head, *page;
 
 		/*
 		 * Similar to the PMD case below, NUMA hinting must take slow
@@ -1109,15 +1109,17 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
 
 		VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
 		page = pte_page(pte);
+		head = compound_head(page);
 
-		if (!page_cache_get_speculative(page))
+		if (!page_cache_get_speculative(head))
 			goto pte_unmap;
 
 		if (unlikely(pte_val(pte) != pte_val(*ptep))) {
-			put_page(page);
+			put_page(head);
 			goto pte_unmap;
 		}
 
+		VM_BUG_ON_PAGE(compound_head(page) != head, page);
 		pages[*nr] = page;
 		(*nr)++;
 
-- 
2.5.0

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