On Tue, Dec 17, 2013 at 01:59:04PM +0800, Xiao Guangrong wrote: > > CCed KVM guys. > > On 05/10/2013 01:11 PM, Stefan Hajnoczi wrote: > > On Fri, May 10, 2013 at 4:28 AM, wenchao <wenchaolinux@xxxxxxxxx> wrote: > >> 于 2013-5-9 22:13, Mel Gorman 写道: > >> > >>> On Thu, May 09, 2013 at 05:50:05PM +0800, wenchaolinux@xxxxxxxxx wrote: > >>>> > >>>> From: Wenchao Xia <wenchaolinux@xxxxxxxxx> > >>>> > >>>> This serial try to enable mremap syscall to cow some private memory > >>>> region, > >>>> just like what fork() did. As a result, user space application would got > >>>> a > >>>> mirror of those region, and it can be used as a snapshot for further > >>>> processing. > >>>> > >>> > >>> What not just fork()? Even if the application was threaded it should be > >>> managable to handle fork just for processing the private memory region > >>> in question. I'm having trouble figuring out what sort of application > >>> would require an interface like this. > >>> > >> It have some troubles: parent - child communication, sometimes > >> page copy. > >> I'd like to snapshot qemu guest's RAM, currently solution is: > >> 1) fork() > >> 2) pipe guest RAM data from child to parent. > >> 3) parent write down the contents. > >> > >> To avoid complex communication for data control, and file content > >> protecting, So let parent instead of child handling the data with > >> a pipe, but this brings additional copy(). I think an explicit API > >> cow mapping an memory region inside one process, could avoid it, > >> and faster and cow less pages, also make user space code nicer. > > > > A new Linux-specific API is not portable and not available on existing > > hosts. Since QEMU supports non-Linux host operating systems the > > fork() approach is preferable. > > > > If you're worried about the memory copy - which should be benchmarked > > - then vmsplice(2) can be used in the child process and splice(2) can > > be used in the parent. It probably doesn't help though since QEMU > > scans RAM pages to find all-zero pages before sending them over the > > socket, and at that point the memory copy might not make much > > difference. > > > > Perhaps other applications can use this new flag better, but for QEMU > > I think fork()'s portability is more important than the convenience of > > accessing the CoW pages in the same process. > > Yup, I agree with you that the new syscall sometimes is not a good solution. > > Currently, we're working on live-update[1] that will be enabled on Qemu firstly, > this feature let the guest run on the new Qemu binary smoothly without > restart, it's good for us to do security-update. > > In this case, we need to move the guest memory on old qemu instance to the > new one, fork() can not help because we need to exec() a new instance, after > that all memory mapping will be destroyed. > > We tried to enable SPLICE_F_MOVE[2] for vmsplice() to move the memory without > memory-copy but the performance isn't so good as we expected: it's due to > some limitations: the page-size, lock, message-size limitation on pipe, etc. > Of course, we will continue to improve this, but wenchao's patch seems a new > direction for us. > > To coordinate with your fork() approach, maybe we can introduce a new flag > for VMA, something like: VM_KEEP_ONEXEC, to tell exec() to do not destroy > this VMA. How about this or you guy have new idea? Really appreciate for your > suggestion. > > [1] http://marc.info/?l=qemu-devel&m=138597598700844&w=2 > [2] https://lkml.org/lkml/2013/10/25/285 Hi, What is the purpose of snapshotting guest RAM here, in the context of local migration? -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
