Commit ad33bb04 authored Feb 26, 2016 by Andrea Arcangeli Committed by Linus Torvalds Feb 27, 2016

mm: thp: fix SMP race condition between THP page fault and MADV_DONTNEED



pmd_trans_unstable()/pmd_none_or_trans_huge_or_clear_bad() were
introduced to locklessy (but atomically) detect when a pmd is a regular
(stable) pmd or when the pmd is unstable and can infinitely transition
from pmd_none() and pmd_trans_huge() from under us, while only holding
the mmap_sem for reading (for writing not).

While holding the mmap_sem only for reading, MADV_DONTNEED can run from
under us and so before we can assume the pmd to be a regular stable pmd
we need to compare it against pmd_none() and pmd_trans_huge() in an
atomic way, with pmd_trans_unstable().  The old pmd_trans_huge() left a
tiny window for a race.

Useful applications are unlikely to notice the difference as doing
MADV_DONTNEED concurrently with a page fault would lead to undefined
behavior.

[akpm@linux-foundation.org: tidy up comment grammar/layout]
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Reported-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 29a9faa6

mm/memory.c

+12 −2

Original line number	Original line	Diff line number	Diff line
	@@ -3404,8 +3404,18 @@ static int __handle_mm_fault(struct mm_struct mm, struct vm_area_struct vma,
	if (unlikely(pmd_none(*pmd)) &&		if (unlikely(pmd_none(*pmd)) &&
	unlikely(__pte_alloc(mm, vma, pmd, address)))		unlikely(__pte_alloc(mm, vma, pmd, address)))
	return VM_FAULT_OOM;		return VM_FAULT_OOM;
	/* if an huge pmd materialized from under us just retry later */		/*
	if (unlikely(pmd_trans_huge(pmd) \|\| pmd_devmap(pmd)))		* If a huge pmd materialized under us just retry later. Use
			* pmd_trans_unstable() instead of pmd_trans_huge() to ensure the pmd
			* didn't become pmd_trans_huge under us and then back to pmd_none, as
			* a result of MADV_DONTNEED running immediately after a huge pmd fault
			* in a different thread of this mm, in turn leading to a misleading
			* pmd_trans_huge() retval. All we have to ensure is that it is a
			* regular pmd that we can walk with pte_offset_map() and we can do that
			* through an atomic read in C, which is what pmd_trans_unstable()
			* provides.
			*/
			if (unlikely(pmd_trans_unstable(pmd) \|\| pmd_devmap(*pmd)))
	return 0;		return 0;
	/*		/*
	* A regular pmd is established and it can't morph into a huge pmd		* A regular pmd is established and it can't morph into a huge pmd

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