Re: pv_ops & gntdev?

[Gerd Hoffmann <kraxel@redhat.com>]

Jeremy Fitzhardinge wrote:
> Gerd Hoffmann wrote:
>> Unmapping the grants when the kernel is about to zap the page range.
>> I think it could be easier because I don't have to link the low-level
>> paravirt code with the gntdev driver then.  Also I can easily batch
>> unmaps.  And there is a performance hit for the extra checks only for
>> processes which actually use grants.
>>   
> 
> Would the unmap operation on the vma be enough to do that?   Hm, I guess
> vm_operations_struct needs something that gets called before the zap
> rather than after (->close).

mmu motifier will fit the bill and additionally work fine on partial unmaps.

>> The (unclean) exit case might be tricky though.  Could be the kernel
>> tries to unpin before zapping all mappings, so we don't trap into xen
>> all the time.
> 
> Yes, the unpin needs to make sure it clears out all the grant refs,
> because you just can't unpin otherwise.
> 
> The alternative is to hook into the fault handler so that a
> fault-n-emulate operation on the pagetable which fails because of the
> grant reference can be fixed up at that point.  But that might be even
> more intrusive.

Or just allow to flag "skip the unpin please for this mm".  It's an
optimization after all, it isn't required, right?

cheers,
  Gerd