From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758062Ab2DJBgQ (ORCPT ); Mon, 9 Apr 2012 21:36:16 -0400 Received: from mx1.redhat.com ([209.132.183.28]:33078 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754130Ab2DJBgP (ORCPT ); Mon, 9 Apr 2012 21:36:15 -0400 Date: Tue, 10 Apr 2012 03:35:49 +0200 From: Oleg Nesterov To: Hugh Dickins Cc: Konstantin Khlebnikov , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Roland Dreier , Andrew Morton , Linus Torvalds Subject: Re: [PATCH RFC] mm: account VMA before forced-COW via /proc/pid/mem Message-ID: <20120410013549.GA19314@redhat.com> References: <20120402153631.5101.44091.stgit@zurg> <20120403143752.GA5150@redhat.com> <4F7C1B67.6030300@openvz.org> <20120404154148.GA7105@redhat.com> <4F7D5859.5050106@openvz.org> <20120407173318.GA5076@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/09, Hugh Dickins wrote: > > Let me reiterate here that I was off at a tangent in bringing this up, > so sorry for any confusion I spread. I guess it was me who added the confusion ;) > > OTOH, if the file was opened without FMODE_WRITE, then I do not > > really understand how (PROT_READ, MAP_SHARED) differs from > > (PROT_READ, MAP_PRIVATE). I meant, from gup(FOLL_FORCE|FOLL_WRITE) pov. I didn't mean mprotect/etc. > The strange weird confusing part is that having checked that you have > permission to write to the file, it then avoids doing so (unless the > area currently has PROT_WRITE): it COWs pages for you instead, > leaving unexpected anon pages in the shared area. Yes, and we could do the same in (PROT_READ, MAP_SHARED) case. This is what looks strange to me. We require PROT_WRITE to force- cow, although we are not going (and shouldn't) write to the file. But, to avoid even more confusion, I am not arguing with your "limit the damage by making GUP write,force fail in that case" suggestion. At least I do not think ptrace/gdb can suffer. > > Speaking of the difference above, I'd wish I could understand > > what VM_MAYSHARE actually means except "MAP_SHARED was used". > > That's precisely it: so it's very useful in /proc/pid/maps, for > deciding whether to show an 's' or a 'p', but not so often when > real decisions are made (where, as you've observed, private readonly > and shared readonly are treated very similarly, without VM_SHARED). Aha, thanks a lot. Oleg.