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

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Date:	Fri, 19 Jun 2015 10:05:46 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Igor Mammedov <imammedo@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, andrey@...l.ru
Subject: Re: [PATCH 3/5] vhost: support upto 509 memory regions

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
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