Messages in this thread | | | Date | Thu, 18 Jun 2015 18:02:46 +0200 | From | Paolo Bonzini <> | Subject | Re: [PATCH 3/5] vhost: support upto 509 memory regions |
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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 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
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