From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk1-f200.google.com (mail-qk1-f200.google.com [209.85.222.200]) by kanga.kvack.org (Postfix) with ESMTP id 290AB8E0001 for ; Tue, 22 Jan 2019 21:43:49 -0500 (EST) Received: by mail-qk1-f200.google.com with SMTP id a199so666069qkb.23 for ; Tue, 22 Jan 2019 18:43:49 -0800 (PST) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id x1si549996qkc.167.2019.01.22.18.43.48 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 22 Jan 2019 18:43:48 -0800 (PST) Date: Tue, 22 Jan 2019 21:43:38 -0500 From: Jerome Glisse Subject: Re: [PATCH RFC 06/24] userfaultfd: wp: support write protection for userfault vma range Message-ID: <20190123024338.GB3652@redhat.com> References: <20190121075722.7945-1-peterx@redhat.com> <20190121075722.7945-7-peterx@redhat.com> <20190121140535.GD3344@redhat.com> <20190122093935.GF14907@xz-x1> <20190122170223.GC3188@redhat.com> <20190123021745.GB2970@xz-x1> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20190123021745.GB2970@xz-x1> Sender: owner-linux-mm@kvack.org List-ID: To: Peter Xu Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hugh Dickins , Maya Gokhale , Johannes Weiner , Martin Cracauer , Denis Plotnikov , Shaohua Li , Andrea Arcangeli , Pavel Emelyanov , Mike Kravetz , Marty McFadden , Mike Rapoport , Mel Gorman , "Kirill A . Shutemov" , "Dr . David Alan Gilbert" , Rik van Riel On Wed, Jan 23, 2019 at 10:17:45AM +0800, Peter Xu wrote: > On Tue, Jan 22, 2019 at 12:02:24PM -0500, Jerome Glisse wrote: > > On Tue, Jan 22, 2019 at 05:39:35PM +0800, Peter Xu wrote: > > > On Mon, Jan 21, 2019 at 09:05:35AM -0500, Jerome Glisse wrote: > > > > > > [...] > > > > > > > > + change_protection(dst_vma, start, start + len, newprot, > > > > > + !enable_wp, 0); > > > > > > > > So setting dirty_accountable bring us to that code in mprotect.c: > > > > > > > > if (dirty_accountable && pte_dirty(ptent) && > > > > (pte_soft_dirty(ptent) || > > > > !(vma->vm_flags & VM_SOFTDIRTY))) { > > > > ptent = pte_mkwrite(ptent); > > > > } > > > > > > > > My understanding is that you want to set write flag when enable_wp > > > > is false and you want to set the write flag unconditionaly, right ? > > > > > > Right. > > > > > > > > > > > If so then you should really move the change_protection() flags > > > > patch before this patch and add a flag for setting pte write flags. > > > > > > > > Otherwise the above is broken at it will only set the write flag > > > > for pte that were dirty and i am guessing so far you always were > > > > lucky because pte were all dirty (change_protection will preserve > > > > dirtyness) when you write protected them. > > > > > > > > So i believe the above is broken or at very least unclear if what > > > > you really want is to only set write flag to pte that have the > > > > dirty flag set. > > > > > > You are right, if we build the tree until this patch it won't work for > > > all the cases. It'll only work if the page was at least writable > > > before and also it's dirty (as you explained). Sorry to be unclear > > > about this, maybe I should at least mention that in the commit message > > > but I totally forgot it. > > > > > > All these problems are solved in later on patches, please feel free to > > > have a look at: > > > > > > mm: merge parameters for change_protection() > > > userfaultfd: wp: apply _PAGE_UFFD_WP bit > > > userfaultfd: wp: handle COW properly for uffd-wp > > > > > > Note that even in the follow up patches IMHO we can't directly change > > > the write permission since the page can be shared by other processes > > > (e.g., the zero page or COW pages). But the general idea is the same > > > as you explained. > > > > > > I tried to avoid squashing these stuff altogether as explained > > > previously. Also, this patch can be seen as a standalone patch to > > > introduce the new interface which seems to make sense too, and it is > > > indeed still working in many cases so I see the latter patches as > > > enhancement of this one. Please let me know if you still want me to > > > have all these stuff squashed, or if you'd like me to squash some of > > > them. > > > > Yeah i have look at those after looking at this one. You should just > > re-order the patch this one first and then one that add new flag, > > then ones that add the new userfaultfd feature. Otherwise you are > > adding a userfaultfd feature that is broken midway ie it is added > > broken and then you fix it. Some one bisecting thing might get hurt > > by that. It is better to add and change everything you need and then > > add the new feature so that the new feature will work as intended. > > > > So no squashing just change the order ie add the userfaultfd code > > last. > > Yes this makes sense, I'll do that in v2. Thanks for the suggestion! Note before doing a v2 i would really like to see some proof of why you need new page table flag see my reply to: userfaultfd: wp: add WP pagetable tracking to x86 As i believe you can identify COW or KSM from UFD write protect with- out a pte flag. Cheers, Jérôme