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

[Paolo Bonzini]

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

>>                                                                        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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