From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk1-f198.google.com (mail-qk1-f198.google.com [209.85.222.198]) by kanga.kvack.org (Postfix) with ESMTP id 7B8668E00A6 for ; Thu, 24 Jan 2019 22:30:56 -0500 (EST) Received: by mail-qk1-f198.google.com with SMTP id y27so7852414qkj.21 for ; Thu, 24 Jan 2019 19:30:56 -0800 (PST) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id h123si17894013qkf.66.2019.01.24.19.30.54 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 24 Jan 2019 19:30:55 -0800 (PST) Date: Fri, 25 Jan 2019 11:30:44 +0800 From: Peter Xu Subject: Re: [PATCH RFC 10/24] userfaultfd: wp: add WP pagetable tracking to x86 Message-ID: <20190125033044.GP18231@xz-x1> References: <20190121075722.7945-1-peterx@redhat.com> <20190121075722.7945-11-peterx@redhat.com> <20190121150937.GE3344@redhat.com> <20190124051616.GE18231@xz-x1> <20190124154050.GC5030@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190124154050.GC5030@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" On Thu, Jan 24, 2019 at 10:40:50AM -0500, Jerome Glisse wrote: > On Thu, Jan 24, 2019 at 01:16:16PM +0800, Peter Xu wrote: > > On Mon, Jan 21, 2019 at 10:09:38AM -0500, Jerome Glisse wrote: > > > On Mon, Jan 21, 2019 at 03:57:08PM +0800, Peter Xu wrote: > > > > From: Andrea Arcangeli > > > > > > > > Accurate userfaultfd WP tracking is possible by tracking exactly which > > > > virtual memory ranges were writeprotected by userland. We can't relay > > > > only on the RW bit of the mapped pagetable because that information is > > > > destroyed by fork() or KSM or swap. If we were to relay on that, we'd > > > > need to stay on the safe side and generate false positive wp faults > > > > for every swapped out page. > > > > (I'm trying to leave comments with my own understanding here; they > > might not be the original purposes when Andrea proposed the idea. > > Andrea, please feel free to chim in anytime especially if I am > > wrong... :-) > > > > > > > > So you want to forward write fault (of a protected range) to user space > > > only if page is not write protected because of fork(), KSM or swap. > > > > > > This write protection feature is only for anonymous page right ? Other- > > > wise how would you protect a share page (ie anyone can look it up and > > > call page_mkwrite on it and start writting to it) ? > > > > AFAIU we want to support shared memory too in the future. One example > > I can think of is current QEMU usage with DPDK: we have two processes > > sharing the guest memory range. So indeed this might not work if > > there are unknown/malicious users of the shared memory, however in > > many use cases the users are all known and AFAIU we should just write > > protect all these users then we'll still get notified when any of them > > write to a page. > > > > > > > > So for anonymous page for fork() the mapcount will tell you if page is > > > write protected for COW. For KSM it is easy check the page flag. > > > > Yes I agree that KSM should be easy. But for COW, please consider > > when we write protect a page that was shared and RW removed due to > > COW. Then when we page fault on this page should we report to the > > monitor? IMHO we can't know if without a specific bit in the PTE. > > > > > > > > For swap you can use the page lock to synchronize. A page that is > > > write protected because of swap is write protected because it is being > > > write to disk thus either under page lock, or with PageWriteback() > > > returning true while write is on going. > > > > For swap I think the major problem is when the page was swapped out of > > main memory and then we write to the page (which was already a swap > > entry now). Then we'll first swap in the page into main memory again, > > but then IMHO we will face the similar issue like COW above - we can't > > judge whether this page was write protected by uffd-wp at all. Of > > course here we can detect the VMA flags and assuming it's write > > protected if the UFFD_WP flag was set on the VMA flag, however we'll > > also mark those pages which were not write protected at all hence > > it'll generate false positives of write protection messages. This > > idea can apply too to above COW use case. As a conclusion, in these > > use cases we should not be able to identify explicitly on page > > granularity write protection if without a specific _PAGE_UFFD_WP bit > > in the PTE entries. > > So i need to think a bit more on this, probably not right now > but just so i get the chain of event properly: > 1 - user space ioctl UFD to write protect a range > 2 - UFD set a flag on the vma and update CPU page table A trivial supplement to these two steps to be clear: the change to VMA flags and PTE permissions are different steps. Say, to write protect a newly mmap()ed region, we need to do: (a) ioctl UFFDIO_REGISTER upon the range: this will properly attach the VM_UFFD_WP flag upon the VMA object, and... (b) ioctl UFFDIO_WRITEPROTECT upon the range again: this will properly apply the new uffd-wp bit and write protect the PTEs/PMDs. Note that the range specified in step (b) could also be part of the buffer, so it does not need to cover the whole VMA, and it's in page granularity. > 3 - page can be individualy write faulted and it sends a > signal to UFD listener and they handle the fault > 4 - UFD kernel update the page table once userspace have > handled the fault and sent result to UFD. At this point > the vma still has the UFD write protect flag set. Yes. As explained above, the VMA can have the VM_UFFD_WP flag even if none of the PTEs underneath was write protected. > > So at any point in time in a range you might have writeable > pte that correspond to already handled UFD write fault. Now > if COW,KSM or swap happens on those then on the next write > fault you do not want to send a signal to userspace but handle > the fault just as usual ? Yes, if the PTE has already resolved the uffd write protection and then it will be just like a normal PTE, because when resolving the uffd-wp page fault we'll also remove the special uffd-wp bit on the PTE/PMD. And IMHO actually what's more special here is when we write protect a shared private page that is for COW (I'll skip KSM since it looks very like this case IIUC): here due to COW the PTE already lost the RW bit, and here when we do uffd-wp upon this page we'll simply apply the uffd-wp bit only to mark that this PTE was especially write protected by userfaults. And when we want to resolve the uffd-wp for such a PTE we'll first try to do COW if it is shared by others by checking against page_mapcount(). > > I believe this is the event flow, so i will ponder on this some > more :) Yes please. :) The workflow of the new ioctl()s was also mentioned in the cover letter. Please feel free to have a look too. Thanks, -- Peter Xu