From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wg0-f47.google.com (mail-wg0-f47.google.com [74.125.82.47]) by kanga.kvack.org (Postfix) with ESMTP id 592406B0035 for ; Sat, 23 Aug 2014 19:56:11 -0400 (EDT) Received: by mail-wg0-f47.google.com with SMTP id b13so11553869wgh.6 for ; Sat, 23 Aug 2014 16:56:10 -0700 (PDT) Received: from jenni2.inet.fi (mta-out1.inet.fi. [62.71.2.228]) by mx.google.com with ESMTP id ot9si49594734wjc.62.2014.08.23.16.56.09 for ; Sat, 23 Aug 2014 16:56:10 -0700 (PDT) Date: Sun, 24 Aug 2014 02:50:58 +0300 From: "Kirill A. Shutemov" Subject: Re: [PATCH v2 1/3] mm: softdirty: enable write notifications on VMAs after VM_SOFTDIRTY cleared Message-ID: <20140823235058.GA27234@node.dhcp.inet.fi> References: <1408571182-28750-1-git-send-email-pfeiner@google.com> <1408831921-10168-1-git-send-email-pfeiner@google.com> <1408831921-10168-2-git-send-email-pfeiner@google.com> <20140823230011.GA26483@node.dhcp.inet.fi> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140823230011.GA26483@node.dhcp.inet.fi> Sender: owner-linux-mm@kvack.org List-ID: To: Peter Feiner Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Cyrill Gorcunov , Pavel Emelyanov , Jamie Liu , Hugh Dickins , Naoya Horiguchi , Andrew Morton On Sun, Aug 24, 2014 at 02:00:11AM +0300, Kirill A. Shutemov wrote: > On Sat, Aug 23, 2014 at 06:11:59PM -0400, Peter Feiner wrote: > > For VMAs that don't want write notifications, PTEs created for read > > faults have their write bit set. If the read fault happens after > > VM_SOFTDIRTY is cleared, then the PTE's softdirty bit will remain > > clear after subsequent writes. > > > > Here's a simple code snippet to demonstrate the bug: > > > > char* m = mmap(NULL, getpagesize(), PROT_READ | PROT_WRITE, > > MAP_ANONYMOUS | MAP_SHARED, -1, 0); > > system("echo 4 > /proc/$PPID/clear_refs"); /* clear VM_SOFTDIRTY */ > > assert(*m == '\0'); /* new PTE allows write access */ > > assert(!soft_dirty(x)); > > *m = 'x'; /* should dirty the page */ > > assert(soft_dirty(x)); /* fails */ > > > > With this patch, write notifications are enabled when VM_SOFTDIRTY is > > cleared. Furthermore, to avoid faults, write notifications are > > disabled when VM_SOFTDIRTY is reset. > > > > Signed-off-by: Peter Feiner One more case to consider: mprotect() which doesn't trigger successful vma_merge() will not set VM_SOFTDIRTY and will not enable write-protect on the vma. It's probably better to take VM_SOFTDIRTY into account in vma_wants_writenotify() and re-think logic in other corners. -- Kirill A. Shutemov -- 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