Commit cbef8478 authored Feb 11, 2015 by Naoya Horiguchi Committed by Linus Torvalds Feb 11, 2015

mm/hugetlb: pmd_huge() returns true for non-present hugepage



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: 290408d4 ("hugetlb: hugepage migration core")
Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Rik van Riel <riel@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>
Cc: Lee Schermerhorn <lee.schermerhorn@hp.com>
Cc: Steve Capper <steve.capper@linaro.org>
Cc: <stable@vger.kernel.org>	[2.6.36+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 61f77eda

arch/x86/mm/gup.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -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

arch/x86/mm/hugetlbpage.c

+7 −1

Original line number	Diff line number	Diff line
		@@ -54,9 +54,15 @@ int pud_huge(pud_t pud)

		#else

		/*
		* 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)

mm/hugetlb.c

+2 −0

Original line number	Diff line number	Diff line
		@@ -3679,6 +3679,8 @@ 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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