From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754536AbaJGPzY (ORCPT ); Tue, 7 Oct 2014 11:55:24 -0400 Received: from mail-lb0-f170.google.com ([209.85.217.170]:47243 "EHLO mail-lb0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753862AbaJGPzV (ORCPT ); Tue, 7 Oct 2014 11:55:21 -0400 MIME-Version: 1.0 In-Reply-To: <20141007155247.GD2342@redhat.com> References: <1412356087-16115-1-git-send-email-aarcange@redhat.com> <1412356087-16115-11-git-send-email-aarcange@redhat.com> <20141006085540.GD2336@work-vm> <20141006164156.GA31075@redhat.com> <20141007141913.GC2342@redhat.com> <20141007155247.GD2342@redhat.com> From: Andy Lutomirski Date: Tue, 7 Oct 2014 08:54:58 -0700 Message-ID: Subject: Re: [PATCH 10/17] mm: rmap preparation for remap_anon_pages To: Andrea Arcangeli Cc: Linus Torvalds , "Dr. David Alan Gilbert" , "qemu-devel@nongnu.org Developers" , KVM list , Linux Kernel Mailing List , linux-mm , Linux API , Andres Lagar-Cavilla , Dave Hansen , Paolo Bonzini , Rik van Riel , Mel Gorman , Andrew Morton , Sasha Levin , Hugh Dickins , Peter Feiner , Christopher Covington , Johannes Weiner , Android Kernel Team , Robert Love , Dmitry Adamushko , Neil Brown , Mike Hommey , Taras Glek , Jan Kara , KOSAKI Motohiro , Michel Lespinasse , Minchan Kim , Keith Packard , "Huangpeng (Peter)" , Isaku Yamahata , Anthony Liguori , Stefan Hajnoczi , Wenchao Xia , Andrew Jones , Juan Quintela Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 7, 2014 at 8:52 AM, Andrea Arcangeli wrote: > On Tue, Oct 07, 2014 at 04:19:13PM +0200, Andrea Arcangeli wrote: >> mremap like interface, or file+commands protocol interface. I tend to >> like mremap more, that's why I opted for a remap_anon_pages syscall >> kept orthogonal to the userfaultfd functionality (remap_anon_pages >> could be also used standalone as an accelerated mremap in some >> circumstances) but nothing prevents to just embed the same mechanism > > Sorry for the self followup, but something else comes to mind to > elaborate this further. > > In term of interfaces, the most efficient I could think of to minimize > the enter/exit kernel, would be to append the "source address" of the > data received from the network transport, to the userfaultfd_write() > command (by appending 8 bytes to the wakeup command). Said that, > mixing the mechanism to be notified about userfaults with the > mechanism to resolve an userfault to me looks a complication. I kind > of liked to keep the userfaultfd protocol is very simple and doing > just its thing. The userfaultfd doesn't need to know how the userfault > was resolved, even mremap would work theoretically (until we run out > of vmas). I thought it was simpler to keep it that way. However if we > want to resolve the fault with a "write()" syscall this may be the > most efficient way to do it, as we're already doing a write() into the > pseudofd to wakeup the page fault that contains the destination > address, I just need to append the source address to the wakeup command. > > I probably grossly overestimated the benefits of resolving the > userfault with a zerocopy page move, sorry. So if we entirely drop the > zerocopy behavior and the TLB flush of the old page like you > suggested, the way to keep the userfaultfd mechanism decoupled from > the userfault resolution mechanism would be to implement an > atomic-copy syscall. That would work for SIGBUS userfaults too without > requiring a pseudofd then. It would be enough then to call > mcopy_atomic(userfault_addr,tmp_addr,len) with the only constraints > that len must be a multiple of PAGE_SIZE. Of course mcopy_atomic > wouldn't page fault or call GUP into the destination address (it can't > otherwise the in-flight partial copy would be visible to the process, > breaking the atomicity of the copy), but it would fill in the > pte/trans_huge_pmd with the same strict behavior that remap_anon_pages > currently has (in turn it would by design bypass the VM_USERFAULT > check and be ideal for resolving userfaults). At the risk of asking a possibly useless question, would it make sense to splice data into a userfaultfd? --Andy > > mcopy_atomic could then be also extended to tmpfs and it would work > without requiring the source page to be a tmpfs page too without > having to convert page types on the fly. > > If I add mcopy_atomic, the patch in subject (10/17) can be dropped of > course so it'd be even less intrusive than the current > remap_anon_pages and it would require zero TLB flush during its > runtime (it would just require an atomic copy). > > So should I try to embed a mcopy_atomic inside userfault_write or can > I expose it to userland as a standalone new syscall? Or should I do > something different? Comments? > > Thanks, > Andrea -- Andy Lutomirski AMA Capital Management, LLC