Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751480AbaKLHTq (ORCPT ); Wed, 12 Nov 2014 02:19:46 -0500 Received: from szxga02-in.huawei.com ([119.145.14.65]:62551 "EHLO szxga02-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750738AbaKLHTp (ORCPT ); Wed, 12 Nov 2014 02:19:45 -0500 Message-ID: <546309A9.3030309@huawei.com> Date: Wed, 12 Nov 2014 15:18:01 +0800 From: zhanghailiang User-Agent: Mozilla/5.0 (Windows NT 6.1; rv:31.0) Gecko/20100101 Thunderbird/31.1.1 MIME-Version: 1.0 To: Andrea Arcangeli CC: , , , Andres Lagar-Cavilla , Dave Hansen , Paolo Bonzini , "Rik van Riel" , Mel Gorman , Andy Lutomirski , Andrew Morton , Sasha Levin , Hugh Dickins , Peter Feiner , "Dr. David Alan Gilbert" , 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 Subject: Re: [PATCH 00/17] RFC: userfault v2 References: <1412356087-16115-1-git-send-email-aarcange@redhat.com> <544E1143.1080905@huawei.com> <20141029174607.GK19606@redhat.com> <545221A4.9030606@huawei.com> In-Reply-To: <545221A4.9030606@huawei.com> Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.177.22.69] X-CFilter-Loop: Reflected Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Andrea, Is there any new about this discussion? ;) Will you plan to support 'only wrprotect fault' in the userfault API? Thanks, zhanghailiang On 2014/10/30 19:31, zhanghailiang wrote: > On 2014/10/30 1:46, Andrea Arcangeli wrote: >> Hi Zhanghailiang, >> >> On Mon, Oct 27, 2014 at 05:32:51PM +0800, zhanghailiang wrote: >>> Hi Andrea, >>> >>> Thanks for your hard work on userfault;) >>> >>> This is really a useful API. >>> >>> I want to confirm a question: >>> Can we support distinguishing between writing and reading memory for userfault? >>> That is, we can decide whether writing a page, reading a page or both trigger userfault. >>> >>> I think this will help supporting vhost-scsi,ivshmem for migration, >>> we can trace dirty page in userspace. >>> >>> Actually, i'm trying to relize live memory snapshot based on pre-copy and userfault, >>> but reading memory from migration thread will also trigger userfault. >>> It will be easy to implement live memory snapshot, if we support configuring >>> userfault for writing memory only. >> >> Mail is going to be long enough already so I'll just assume tracking >> dirty memory in userland (instead of doing it in kernel) is worthy >> feature to have here. >> >> After some chat during the KVMForum I've been already thinking it >> could be beneficial for some usage to give userland the information >> about the fault being read or write, combined with the ability of >> mapping pages wrprotected to mcopy_atomic (that would work without >> false positives only with MADV_DONTFORK also set, but it's already set >> in qemu). That will require "vma->vm_flags & VM_USERFAULT" to be >> checked also in the wrprotect faults, not just in the not present >> faults, but it's not a massive change. Returning the read/write >> information is also a not massive change. This will then payoff mostly >> if there's also a way to remove the memory atomically (kind of >> remap_anon_pages). >> >> Would that be enough? I mean are you still ok if non present read >> fault traps too (you'd be notified it's a read) and you get >> notification for both wrprotect and non present faults? >> > Hi Andrea, > > Thanks for your reply, and your patience;) > > Er, maybe i didn't describe clearly. What i really need for live memory snapshot > is only wrprotect fault, like kvm's dirty tracing mechanism, *only tracing write action*. > > My initial solution scheme for live memory snapshot is: > (1) pause VM > (2) using userfaultfd to mark all memory of VM is wrprotect (readonly) > (3) save deivce state to snapshot file > (4) resume VM > (5) snapshot thread begin to save page of memory to snapshot file > (6) VM is going to run, and it is OK for VM or other thread to read ram (no fault trap), > but if VM try to write page (dirty the page), there will be > a userfault trap notification. > (7) a fault-handle-thread reads the page request from userfaultfd, > it will copy content of the page to some buffers, and then remove the page's > wrprotect limit(still using the userfaultfd to tell kernel). > (8) after step (7), VM can continue to write the page which is now can be write. > (9) snapshot thread save the page cached in step (7) > (10) repeat step (5)~(9) until all VM's memory is saved to snapshot file. > > So, what i need for userfault is supporting only wrprotect fault. i don't > want to get notification for non present reading faults, it will influence > VM's performance and the efficiency of doing snapshot. > > Also, i think this feature will benefit for migration of ivshmem and vhost-scsi > which have no dirty-page-tracing now. > >> The question then is how you mark the memory readonly to let the >> wrprotect faults trap if the memory already existed and you didn't map >> it yourself in the guest with mcopy_atomic with a readonly flag. >> >> My current plan would be: >> >> - keep MADV_USERFAULT|NOUSERFAULT just to set VM_USERFAULT for the >> fast path check in the not-present and wrprotect page fault >> >> - if VM_USERFAULT is set, find if there's a userfaultfd registered >> into that vma too >> >> if yes engage userfaultfd protocol >> >> otherwise raise SIGBUS (single threaded apps should be fine with >> SIGBUS and it'll avoid them to spawn a thread in order to talk the >> userfaultfd protocol) >> >> - if userfaultfd protocol is engaged, return read|write fault + fault >> address to read(ufd) syscalls >> >> - leave the "userfault" resolution mechanism independent of the >> userfaultfd protocol so we keep the two problems separated and we >> don't mix them in the same API which makes it even harder to >> finalize it. >> >> add mcopy_atomic (with a flag to map the page readonly too) >> >> The alternative would be to hide mcopy_atomic (and even >> remap_anon_pages in order to "remove" the memory atomically for >> the externalization into the cloud) as userfaultfd commands to >> write into the fd. But then there would be no much point to keep >> MADV_USERFAULT around if I do so and I could just remove it >> too or it doesn't look clean having to open the userfaultfd just >> to issue an hidden mcopy_atomic. >> >> So it becomes a decision if the basic SIGBUS mode for single >> threaded apps should be supported or not. As long as we support >> SIGBUS too and we don't force to use userfaultfd as the only >> mechanism to be notified about userfaults, having a separate >> mcopy_atomic syscall sounds cleaner. >> >> Perhaps mcopy_atomic could be used in other cases that may arise >> later that may not be connected with the userfault. >> >> Questions to double check the above plan is ok: >> >> 1) should I drop the SIGBUS behavior and MADV_USERFAULT? >> >> 2) should I hide mcopy_atomic as a write into the userfaultfd? >> >> NOTE: even if I hide mcopy_atomic as a userfaultfd command to write >> into the fd, the buffer pointer passed to write() syscall would >> still _not_ be pointing to the data like a regular write, but it >> would be a pointer to a command structure that points to the source >> and destination data of the "hidden" mcopy_atomic, the only >> advantage is that perhaps I could wakeup the blocked page faults >> without requiring an additional syscall. >> >> The standalone mcopy_atomic would still require a write into the >> userfaultfd as it happens now after remap_anon_pages returns, in >> order to wakeup the stopped page faults. >> >> 3) should I add a registration command to trap only write faults? >> > > Sure, that is what i really need;) > > > Best Regards， > zhanghailiang > >> The protocol can always be extended later anyway in a backwards >> compatible way but it's better if we get it fully featured from the >> start. >> >> For completeness, some answers for other questions I've seen floating >> around but that weren't posted on the list yet (you can skip reading >> the below part if not interested): >> >> - open("/dev/userfault") instead of sys_userfaultfd(), I don't see the >> benefit: userfaultfd is just like eventfd in terms of kernel API and >> registering a /dev/ device actually sounds trickier. userfault is a >> core VM feature and generally we prefer syscalls for core VM >> features instead of running ioctl on some chardev that may or may >> not exist. (like we did with /dev/ksm -> MADV_MERGEABLE) >> >> - there was a suggestion during KVMForum about allowing an external >> program to attach to any MM. Like ptrace. So you could have a single >> process managing all userfaults for different processes. However >> because I cannot allow multiple userfaultfd to register into the >> same range, this doesn't look very reliable (ptrace is kind of an >> optional/debug feature while if userfault goes wrong and returns >> -EBUSY things go bad) and there may be other complications. If I'd >> allow multiple userfaultfd to register into the same range, I >> wouldn't even know who to deliver the userfault to. It is an erratic >> behavior. Currently it'd return -EBUSY if the app has a bug and does >> that, but maybe later this can be relaxed to allow higher >> scalability with a flag (userfaultfd gets flags as parameters), but >> it still would need to be the same logic that manages userfaults and >> the only point of allowing multiple ufd to map the same range would >> be SMP scalability. So I tend to see the userfaultfd as a MM local >> thing. The thread managing the userfaults can still talk with >> another process in the local machine using pipes or sockets if it >> needs to. >> >> - the userfaultfd protocol version handshake was done this way because >> it looked more reliable. >> >> Of course we could pass the version of the protocol as parameter to >> userfaultfd too, but running the syscall multiple times until >> -EPROTO didn't return anymore doesn't seem any better than writing >> into the fd the wanted protocol until you read it back instead of >> -1ULL. It just looked more reliable not having to run the syscall >> again and again while depending on -EPROTO or some other >> -Esomething. >> >> Thanks, >> Andrea >> >> . >> > > > -- > To unsubscribe from this list: send the line "unsubscribe kvm" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > > . > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/