From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f42.google.com (mail-wm0-f42.google.com [74.125.82.42]) by kanga.kvack.org (Postfix) with ESMTP id 69D546B0009 for ; Tue, 23 Feb 2016 16:18:47 -0500 (EST) Received: by mail-wm0-f42.google.com with SMTP id a4so2653700wme.1 for ; Tue, 23 Feb 2016 13:18:47 -0800 (PST) Received: from mail.linuxfoundation.org (mail.linuxfoundation.org. [140.211.169.12]) by mx.google.com with ESMTPS id uv9si47079768wjc.29.2016.02.23.13.18.46 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 23 Feb 2016 13:18:46 -0800 (PST) Date: Tue, 23 Feb 2016 13:18:44 -0800 From: Andrew Morton Subject: Re: [PATCH 1/1] mm: thp: fix SMP race condition between THP page fault and MADV_DONTNEED Message-Id: <20160223131844.d5d2767a0cc44bd8cbb78221@linux-foundation.org> In-Reply-To: <1456253350-3959-2-git-send-email-aarcange@redhat.com> References: <20160223154950.GA22449@node.shutemov.name> <1456253350-3959-1-git-send-email-aarcange@redhat.com> <1456253350-3959-2-git-send-email-aarcange@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Andrea Arcangeli Cc: "\\\"Kirill A. Shutemov\\\"" , linux-mm@kvack.org, stable@vger.kernel.org On Tue, 23 Feb 2016 19:49:10 +0100 Andrea Arcangeli wrote: > 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. Thanks. I put a cc:stable on this as it appears to be applicable to 4.4 and perhaps earlier. It generates a reject against 4.4 because of the recently-added pmd_devmap() test. It's easily fixed but I don't have a process to handle -stable rejects. This means that when Greg hits the reject he'll ask us for a fixed up version. -- 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