linux-kernel - [PATCH 3.16.y-ckt 098/183] mm/hugetlb: pmd_huge() returns true for non-present hugepage

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Message-Id: <1425635874-19087-99-git-send-email-luis.henriques@canonical.com>
Date:	Fri,  6 Mar 2015 09:56:29 +0000
From:	Luis Henriques <luis.henriques@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Hugh Dickins <hughd@...gle.com>,
	James Hogan <james.hogan@...tec.com>,
	David Rientjes <rientjes@...gle.com>,
	Mel Gorman <mel@....ul.ie>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>, Rik van Riel <riel@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Nishanth Aravamudan <nacc@...ux.vnet.ibm.com>,
	Lee Schermerhorn <lee.schermerhorn@...com>,
	Steve Capper <steve.capper@...aro.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Luis Henriques <luis.henriques@...onical.com>
Subject: [PATCH 3.16.y-ckt 098/183] mm/hugetlb: pmd_huge() returns true for non-present hugepage

3.16.7-ckt8 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Naoya Horiguchi <n-horiguchi@...jp.nec.com>

commit cbef8478bee55775ac312a574aad48af7bb9cf9f upstream.

Migrating hugepages and hwpoisoned hugepages are considered as non-present
hugepages, and they are referenced via migration entries and hwpoison
entries in their page table slots.

This behavior causes race condition because pmd_huge() doesn't tell
non-huge pages from migrating/hwpoisoned hugepages.  follow_page_mask() is
one example where the kernel would call follow_page_pte() for such
hugepage while this function is supposed to handle only normal pages.

To avoid this, this patch makes pmd_huge() return true when pmd_none() is
true *and* pmd_present() is false.  We don't have to worry about mixing up
non-present pmd entry with normal pmd (pointing to leaf level pte entry)
because pmd_present() is true in normal pmd.

The same race condition could happen in (x86-specific) gup_pmd_range(),
where this patch simply adds pmd_present() check instead of pmd_huge().
This is because gup_pmd_range() is fast path.  If we have non-present
hugepage in this function, we will go into gup_huge_pmd(), then return 0
at flag mask check, and finally fall back to the slow path.

Fixes: 290408d4a2 ("hugetlb: hugepage migration core")
Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc: Hugh Dickins <hughd@...gle.com>
Cc: James Hogan <james.hogan@...tec.com>
Cc: David Rientjes <rientjes@...gle.com>
Cc: Mel Gorman <mel@....ul.ie>
Cc: Johannes Weiner <hannes@...xchg.org>
Cc: Michal Hocko <mhocko@...e.cz>
Cc: Rik van Riel <riel@...hat.com>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: Luiz Capitulino <lcapitulino@...hat.com>
Cc: Nishanth Aravamudan <nacc@...ux.vnet.ibm.com>
Cc: Lee Schermerhorn <lee.schermerhorn@...com>
Cc: Steve Capper <steve.capper@...aro.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Luis Henriques <luis.henriques@...onical.com>
---
 arch/x86/mm/gup.c         | 2 +-
 arch/x86/mm/hugetlbpage.c | 8 +++++++-
 mm/hugetlb.c              | 2 ++
 3 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/gup.c b/arch/x86/mm/gup.c
index 207d9aef662d..448ee8912d9b 100644
--- a/arch/x86/mm/gup.c
+++ b/arch/x86/mm/gup.c
@@ -172,7 +172,7 @@ static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
 		 */
 		if (pmd_none(pmd) || pmd_trans_splitting(pmd))
 			return 0;
-		if (unlikely(pmd_large(pmd))) {
+		if (unlikely(pmd_large(pmd) || !pmd_present(pmd))) {
 			/*
 			 * NUMA hinting faults need to be handled in the GUP
 			 * slowpath for accounting purposes and so that they
diff --git a/arch/x86/mm/hugetlbpage.c b/arch/x86/mm/hugetlbpage.c
index 8b977ebf9388..006cc914994b 100644
--- a/arch/x86/mm/hugetlbpage.c
+++ b/arch/x86/mm/hugetlbpage.c
@@ -66,9 +66,15 @@ follow_huge_addr(struct mm_struct *mm, unsigned long address, int write)
 	return ERR_PTR(-EINVAL);
 }
 
+/*
+ * pmd_huge() returns 1 if @pmd is hugetlb related entry, that is normal
+ * hugetlb entry or non-present (migration or hwpoisoned) hugetlb entry.
+ * Otherwise, returns 0.
+ */
 int pmd_huge(pmd_t pmd)
 {
-	return !!(pmd_val(pmd) & _PAGE_PSE);
+	return !pmd_none(pmd) &&
+		(pmd_val(pmd) & (_PAGE_PRESENT|_PAGE_PSE)) != _PAGE_PRESENT;
 }
 
 int pud_huge(pud_t pud)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 7ae54449f252..4a1af45eefcd 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3673,6 +3673,8 @@ follow_huge_pmd(struct mm_struct *mm, unsigned long address,
 {
 	struct page *page;
 
+	if (!pmd_present(*pmd))
+		return NULL;
 	page = pte_page(*(pte_t *)pmd);
 	if (page)
 		page += ((address & ~PMD_MASK) >> PAGE_SHIFT);
--
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