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

[Michael S. Tsirkin]

On Fri, Jun 19, 2015 at 10:52:47AM +0200, Paolo Bonzini wrote:
> 
> 
> On 19/06/2015 10:05, Michael S. Tsirkin wrote:
> > > 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?
> 
> No, because in the meanwhile another DIMM could have been hotplugged
> at the same place where the old one was.  This is legal:
> 
>     user                      guest                 QEMU
>     ----------------------------------------------------------------------------------------
>                               start I/O
>                                   '---------------> address_space_map
>     device_del
>         '-------------------> receives SCI
>                               executes _EJ0
>                                   '---------------> memory_region_del_subregion
>                                                     object_unparent
>     device_add
>         '-----------------------------------------> device_set_realized
>                                                       hotplug_handler_plug
>                                                         pc_machine_device_plug_cb
>                                                           pc_dimm_plug
>                                                             memory_region_add_subregion
> 
>                                                     I/O finishes
>                                                       address_space_unmap
> 
> Surprise removal similarly could be done in QEMU, but it will hold to
> some resources for as long as the device backends need them.
> 
> Paolo

OK so what's the problem with checking for this condition,
after address_space_unmap detects that ref count is 0
and before calling mmap(MAP_NORESERVE)?

At that point we are off the data path so we can take locks.


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