Patch "mm: thp: fix SMP race condition between THP page fault and MADV_DONTNEED" has been added to the 3.14-stable tree

This is a note to let you know that I've just added the patch titled

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

to the 3.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mm-thp-fix-smp-race-condition-between-thp-page-fault-and-madv_dontneed.patch
and it can be found in the queue-3.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Andrea Arcangeli <[email protected]>
Date: Fri, 26 Feb 2016 15:19:28 -0800
Subject: mm: thp: fix SMP race condition between THP page fault and MADV_DONTNEED

From: Andrea Arcangeli <[email protected]>

commit ad33bb04b2a6cee6c1f99fabb15cddbf93ff0433 upstream.

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.

[js] 3.12 backport: no pmd_devmap in 3.12 yet.

---
 mm/memory.c |   14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

@@ -3770,8 +3770,18 @@ static int __handle_mm_fault(struct mm_s
 	if (unlikely(pmd_none(*pmd)) &&
 	    unlikely(__pte_alloc(mm, vma, pmd, address)))
 		return VM_FAULT_OOM;
-	/* if an huge pmd materialized from under us just retry later */
-	if (unlikely(pmd_trans_huge(*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)))
 		return 0;
 	/*
 	 * A regular pmd is established and it can't morph into a huge pmd

Patches currently in stable-queue which might be from [email protected] are

queue-3.14/mm-thp-fix-smp-race-condition-between-thp-page-fault-and-madv_dontneed.patch