From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail333.us4.mandrillapp.com ([205.201.137.77]:42233 "EHLO mail333.us4.mandrillapp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752039AbcCAIP1 (ORCPT ); Tue, 1 Mar 2016 03:15:27 -0500 Received: from pmta03.dal05.mailchimp.com (127.0.0.1) by mail333.us4.mandrillapp.com id hql7pu174noj for ; Tue, 1 Mar 2016 08:15:26 +0000 (envelope-from ) From: Subject: Patch "mm: thp: fix SMP race condition between THP page fault and MADV_DONTNEED" has been added to the 4.4-stable tree To: , , , , Cc: , Message-Id: <1456814212164162@kroah.com> Date: Tue, 01 Mar 2016 08:15:26 +0000 MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: stable-owner@vger.kernel.org List-ID: 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 4.4-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-4.4 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let know about it. >>From ad33bb04b2a6cee6c1f99fabb15cddbf93ff0433 Mon Sep 17 00:00:00 2001 From: Andrea Arcangeli 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 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. [akpm@linux-foundation.org: tidy up comment grammar/layout] Signed-off-by: Andrea Arcangeli Reported-by: Kirill A. Shutemov Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- mm/memory.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) --- a/mm/memory.c +++ b/mm/memory.c @@ -3399,8 +3399,18 @@ 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 aarcange@redhat.com are queue-4.4/mm-thp-fix-smp-race-condition-between-thp-page-fault-and-madv_dontneed.patch