On Thu, Jun 18, 2015 at 06:02:46PM +0200, Paolo Bonzini wrote: > > > On 18/06/2015 16:47, Michael S. Tsirkin wrote: > >> However, with Igor's patches a memory_region_del_subregion will cause a > >> mmap(MAP_NORESERVE), which _does_ have the effect of making the hva go away. > >> > >> I guess one way to do it would be to alias the same page in two places, > >> one for use by vhost and one for use by everything else. However, the > >> kernel does not provide the means to do this kind of aliasing for > >> anonymous mmaps. > > > > Basically pages go away on munmap, so won't simple > > lock > > munmap > > mmap(MAP_NORESERVE) > > unlock > > do the trick? > > Not sure I follow. Here we have this: > > VCPU 1 VCPU 2 I/O worker > ---------------------------------------------------------------------------------------- > take big QEMU lock > p = address_space_map(hva, len) > pass I/O request to worker thread > read(fd, p, len) > release big QEMU lock > > memory_region_del_subregion > mmap(MAP_NORESERVE) > > read returns EFAULT Why doesn't it EFAULT without mmap(MAP_NORESERVE)? Doesn't memory_region_del_subregion free the memory? > wake up VCPU 1 > take big QEMU lock > EFAULT? What's that? > > In another scenario you are less lucky: the memory accesses > between address_space_map/unmap aren't done in the kernel and > you get a plain old SIGSEGV. > > This is not something that you can fix with a lock. The very > purpose of the map/unmap API is to do stuff asynchronously while > the lock is released. > > Thanks, > > Paolo -- 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
