On 19/06/2015 09:56, Michael S. Tsirkin wrote: > 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? No, only destruction of the memory region frees it. address_space_map takes a reference to the memory region and address_space_unmap releases it. Paolo >> 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
