From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from mx1.redhat.com ([209.132.183.28]:24212 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751333AbaDYTJs (ORCPT ); Fri, 25 Apr 2014 15:09:48 -0400 Date: Fri, 25 Apr 2014 21:09:31 +0200 From: Oleg Nesterov To: Hugh Dickins Cc: Linus Torvalds , Andrew Morton , Jan Kara , Roland Dreier , Konstantin Khlebnikov , "Kirill A. Shutemov" , Mauro Carvalho Chehab , Omar Ramirez Luna , Inki Dae , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-rdma@vger.kernel.org, linux-media@vger.kernel.org Subject: Re: [PATCH] mm: get_user_pages(write,force) refuse to COW in shared areas Message-ID: <20140425190931.GA11323@redhat.com> References: <20140424133055.GA13269@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-media-owner@vger.kernel.org List-ID: On 04/24, Hugh Dickins wrote: > > On Thu, 24 Apr 2014, Oleg Nesterov wrote: > > > So, what do you think about the patch below? It is probably fine in any case, > > but is there any "strong" reason to follow the gup's behaviour and forbid the > > anon page in VM_MAYSHARE && !VM_MAYWRITE vma? > > I don't think there is a "strong" reason to forbid it. > > The strongest reason is simply that it's much safer if uprobes follows > the same conventions as mm, and get_user_pages() happens to have > forbidden that all along. > > The philosophical reason to forbid it is that the user mmapped with > MAP_SHARED, and it's merely a kernel-internal detail that we flip off > VM_SHARED and treat these read-only shared mappings very much like > private mappings. The user asked for MAP_SHARED, and we prefer to > respect that by not letting private COWs creep in. > > We could treat those mappings even more like private mappings, and > allow the COWs; but better to be strict about it, so long as doing > so doesn't give you regressions. Great, thanks a lot! I was worried I missed something subtle. And I forgot to mention, there is another reason why I would like to change uprobes to follow the same convention. I still think it would be better to kill __replace_page() and use gup(FOLL_WRITE | FORCE) in uprobe_write_opcode(). > > --- x/kernel/events/uprobes.c > > +++ x/kernel/events/uprobes.c > > @@ -127,12 +127,13 @@ struct xol_area { > > */ > > static bool valid_vma(struct vm_area_struct *vma, bool is_register) > > { > > - vm_flags_t flags = VM_HUGETLB | VM_MAYEXEC | VM_SHARED; > > + vm_flags_t flags = VM_HUGETLB | VM_MAYEXEC; > > I think a one-line patch changing VM_SHARED to VM_MAYSHARE would do it, > wouldn't it? And save you from having to export is_cow_mapping() > from mm/memory.c. (I used is_cow_mapping() because I had to make the > test more complex anyway, just to exclude the case which had been > oddly handled before.) Indeed, thanks! Oleg.