From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-la0-f54.google.com (mail-la0-f54.google.com [209.85.215.54]) by kanga.kvack.org (Postfix) with ESMTP id AC82F6B0036 for ; Tue, 26 Aug 2014 10:56:21 -0400 (EDT) Received: by mail-la0-f54.google.com with SMTP id hz20so15403481lab.27 for ; Tue, 26 Aug 2014 07:56:20 -0700 (PDT) Received: from jenni1.inet.fi (mta-out1.inet.fi. [62.71.2.228]) by mx.google.com with ESMTP id jf6si3916921lac.14.2014.08.26.07.56.19 for ; Tue, 26 Aug 2014 07:56:19 -0700 (PDT) Date: Tue, 26 Aug 2014 17:56:12 +0300 From: "Kirill A. Shutemov" Subject: Re: [PATCH v5] mm: softdirty: enable write notifications on VMAs after VM_SOFTDIRTY cleared Message-ID: <20140826145612.GA11226@node.dhcp.inet.fi> References: <1408571182-28750-1-git-send-email-pfeiner@google.com> <1408937681-1472-1-git-send-email-pfeiner@google.com> <20140826064952.GR25918@moon> <20140826140419.GA10625@node.dhcp.inet.fi> <20140826141914.GA8952@moon> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140826141914.GA8952@moon> Sender: owner-linux-mm@kvack.org List-ID: To: Cyrill Gorcunov Cc: Hugh Dickins , Peter Feiner , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Pavel Emelyanov , Jamie Liu , Naoya Horiguchi , Andrew Morton , Magnus Damm On Tue, Aug 26, 2014 at 06:19:14PM +0400, Cyrill Gorcunov wrote: > On Tue, Aug 26, 2014 at 05:04:19PM +0300, Kirill A. Shutemov wrote: > > > > > > > > But now I'm realizing that if this is the _only_ place which modifies > > > > vm_flags with down_read, then it's "probably" safe. I've a vague > > > > feeling that this was discussed before - is that so, Cyrill? > > > > > > Well, as far as I remember we were not talking before about vm_flags > > > and read-lock in this function, maybe it was on some unrelated lkml thread > > > without me CC'ed? Until I miss something obvious using read-lock here > > > for vm_flags modification should be safe, since the only thing which is > > > important (in context of vma-softdirty) is the vma's presence. Hugh, > > > mind to refresh my memory, how long ago the discussion took place? > > > > It seems safe in vma-softdirty context. But if somebody else will decide that > > it's fine to modify vm_flags without down_write (in their context), we > > will get trouble. Sasha will come with weird bug report one day ;) > > > > At least vm_flags must be updated atomically to avoid race in middle of > > load-modify-store. > > Which race you mean here? Two concurrent clear-refs? Two concurent clear-refs is fine. But if somebody else will exploit the same approch to set/clear other VM_FOO and it will race with clear-refs we get trouble: some modifications can be lost. Basically, it's safe if only soft-dirty is allowed to modify vm_flags without down_write(). But why is soft-dirty so special? Should we consider moving protection of some vma fields under per-vma lock rather use over-loaded mmap_sem? -- 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