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

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Date:	Thu, 18 Jun 2015 18:02:46 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Michael S. Tsirkin" <mst@...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 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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