Re: [PATCH 3/5] vhost: support upto 509 memory regions

Michael S. Tsirkin
2015-06-19T08:06:01Z

On Fri, Jun 19, 2015 at 09:57:22AM +0200, Paolo Bonzini wrote:
>
>
> 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

Confused. So can we call mmap(MAP_NORESERVE) in address_space_unmap
after we detect refcount is 0?

> >> 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