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

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Date:	Fri, 19 Jun 2015 10:52:47 +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 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
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