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. Stefan -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href