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

Michael S. Tsirkin
2015-06-19T07:56:11Z

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