From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk0-f182.google.com (mail-qk0-f182.google.com [209.85.220.182]) by kanga.kvack.org (Postfix) with ESMTP id 5F0656B0009 for ; Tue, 23 Feb 2016 13:49:14 -0500 (EST) Received: by mail-qk0-f182.google.com with SMTP id x1so71691907qkc.1 for ; Tue, 23 Feb 2016 10:49:14 -0800 (PST) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id d21si35127266qkb.86.2016.02.23.10.49.13 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 23 Feb 2016 10:49:13 -0800 (PST) From: Andrea Arcangeli Subject: [PATCH 1/1] mm: thp: fix SMP race condition between THP page fault and MADV_DONTNEED Date: Tue, 23 Feb 2016 19:49:10 +0100 Message-Id: <1456253350-3959-2-git-send-email-aarcange@redhat.com> In-Reply-To: <1456253350-3959-1-git-send-email-aarcange@redhat.com> References: <20160223154950.GA22449@node.shutemov.name> <1456253350-3959-1-git-send-email-aarcange@redhat.com> Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton , "\\\"Kirill A. Shutemov\\\"" Cc: linux-mm@kvack.org 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. Signed-off-by: Andrea Arcangeli Reported-by: Kirill A. Shutemov --- mm/memory.c | 15 +++++++++++++-- 1 file changed, 13 insertions(+), 2 deletions(-) diff --git a/mm/memory.c b/mm/memory.c index 635451a..50347ed 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -3404,8 +3404,19 @@ static int __handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma, 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) || pmd_devmap(*pmd))) + /* + * If an huge pmd materialized from under us just retry later. + * Use pmd_trans_unstable() instead of pmd_trans_huge() to + * ensure the pmd didn't become pmd_trans_huge from under us + * and then back to pmd_none, as result of MADV_DONTNEED + * running immediately after a huge pmd fault of 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() is provided for. + */ + if (unlikely(pmd_trans_unstable(pmd) || pmd_devmap(*pmd))) return 0; /* * A regular pmd is established and it can't morph into a huge pmd -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org