linux-kernel - [PATCH 3/8] mm: handle PTE-mapped tail pages in gerneric fast gup implementaiton

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Date:   Tue, 23 Jul 2019 16:38:26 +0530
From:   Ajay Kaher <akaher@...are.com>
To:     <gregkh@...uxfoundation.org>
CC:     <torvalds@...ux-foundation.org>, <aarcange@...hat.com>,
        <hughd@...gle.com>, <dave.hansen@...el.com>, <mgorman@...e.de>,
        <riel@...hat.com>, <mhocko@...e.cz>, <jannh@...gle.com>,
        <linux-kernel@...r.kernel.org>, <stable@...nel.org>,
        <stable@...r.kernel.org>, <srivatsab@...are.com>,
        <srivatsa@...il.mit.edu>, <amakhalov@...are.com>,
        <srinidhir@...are.com>, <bvikas@...are.com>, <srostedt@...are.com>,
        <akaher@...are.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Steve Capper <steve.capper@...aro.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH 3/8] mm: handle PTE-mapped tail pages in gerneric fast gup implementaiton

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

commit 7aef4172c7957d7e65fc172be4c99becaef855d4 upstream.

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>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: Hugh Dickins <hughd@...gle.com>
Cc: Dave Hansen <dave.hansen@...el.com>
Cc: Mel Gorman <mgorman@...e.de>
Cc: Rik van Riel <riel@...hat.com>
Cc: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc: Steve Capper <steve.capper@...aro.org>
Cc: Johannes Weiner <hannes@...xchg.org>
Cc: Michal Hocko <mhocko@...e.cz>
Cc: Christoph Lameter <cl@...ux.com>
Cc: David Rientjes <rientjes@...gle.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Ajay Kaher <akaher@...are.com>
---
 mm/gup.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 2cd3b31..45c544b 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1070,7 +1070,7 @@ 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
@@ -1082,15 +1082,17 @@ 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)++;
 
-- 
2.7.4