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

[Michael S. Tsirkin]

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
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