From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk1-f197.google.com (mail-qk1-f197.google.com [209.85.222.197]) by kanga.kvack.org (Postfix) with ESMTP id 0587D8E0001 for ; Tue, 22 Jan 2019 21:17:57 -0500 (EST) Received: by mail-qk1-f197.google.com with SMTP id c71so632741qke.18 for ; Tue, 22 Jan 2019 18:17:56 -0800 (PST) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id c26si5809859qtm.398.2019.01.22.18.17.56 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 22 Jan 2019 18:17:56 -0800 (PST) Date: Wed, 23 Jan 2019 10:17:45 +0800 From: Peter Xu Subject: Re: [PATCH RFC 06/24] userfaultfd: wp: support write protection for userfault vma range Message-ID: <20190123021745.GB2970@xz-x1> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190122170223.GC3188@redhat.com> Sender: owner-linux-mm@kvack.org List-ID: To: Jerome Glisse 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 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! -- Peter Xu