Re: [PATCH v6 14/15] qmp: Include "reserve" property of memory backends

[David Hildenbrand <david@redhat.com>]

On 23.04.21 13:21, Markus Armbruster wrote:
> David Hildenbrand <david@redhat.com> writes:
> 
>> On 23.04.21 13:00, Markus Armbruster wrote:
>>> David Hildenbrand <david@redhat.com> writes:
>>>
>>>> Let's include the new property.
>>>>
>>>> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
>>>> Cc: Eric Blake <eblake@redhat.com>
>>>> Cc: Markus Armbruster <armbru@redhat.com>
>>>> Cc: Igor Mammedov <imammedo@redhat.com>
>>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>>> ---
>>>>    hw/core/machine-qmp-cmds.c | 1 +
>>>>    qapi/machine.json          | 4 ++++
>>>>    2 files changed, 5 insertions(+)
>>>>
>>>> diff --git a/hw/core/machine-qmp-cmds.c b/hw/core/machine-qmp-cmds.c
>>>> index d41db5b93b..2d135ecdd0 100644
>>>> --- a/hw/core/machine-qmp-cmds.c
>>>> +++ b/hw/core/machine-qmp-cmds.c
>>>> @@ -175,6 +175,7 @@ static int query_memdev(Object *obj, void *opaque)
>>>>            m->dump = object_property_get_bool(obj, "dump", &error_abort);
>>>>            m->prealloc = object_property_get_bool(obj, "prealloc", &error_abort);
>>>>            m->share = object_property_get_bool(obj, "share", &error_abort);
>>>> +        m->reserve = object_property_get_bool(obj, "reserve", &error_abort);
>>>>            m->policy = object_property_get_enum(obj, "policy", "HostMemPolicy",
>>>>                                                 &error_abort);
>>>>            host_nodes = object_property_get_qobject(obj,
>>>> diff --git a/qapi/machine.json b/qapi/machine.json
>>>> index 32650bfe9e..5932139d20 100644
>>>> --- a/qapi/machine.json
>>>> +++ b/qapi/machine.json
>>>> @@ -798,6 +798,9 @@
>>>>    #
>>>>    # @share: whether memory is private to QEMU or shared (since 6.1)
>>>>    #
>>>> +# @reserve: whether swap space (or huge pages) was reserved if applicable
>>>> +#           (since 6.1)
>>>> +#
>>>>    # @host-nodes: host nodes for its memory policy
>>>>    #
>>>>    # @policy: memory policy of memory backend
>>>> @@ -812,6 +815,7 @@
>>>>        'dump':       'bool',
>>>>        'prealloc':   'bool',
>>>>        'share':      'bool',
>>>> +    'reserve':    'bool',
>>>>        'host-nodes': ['uint16'],
>>>>        'policy':     'HostMemPolicy' }}
>>>
>>> Double-checking: true means definitely reserved, and false means
>>> definitely not reserved.  Correct?
>>
>> True means "reserved if applicable" which means "not reserved if not
>> applicable". False means "definitely not reserved".
>>
>> (any recommendation how to rephrase are appreciated; I tried my best --
>> this interface here makes it especially hard -- it's easier for the
>> property itself)
> 
> When is it "applicable"?
> 

When the OS supports it for the memory type and it hasn't been disabled.

Linux handling as described in
   [PATCH v6 09/15] util/mmap-alloc: Support RAM_NORESERVE via
   MAP_NORESERVE under Linux
and

   https://www.kernel.org/doc/Documentation/vm/overcommit-accounting

Summary *without* MAP_NORESERVE:

a) !Hugetlbfs with Memory overcommit disabled via
     ("/proc/sys/vm/overcommit_memory == 2"): never

b) !Hugetlbfs with Memory overcommit enabled
   1) Shared mappings on files: never

   2) Private mappings on files: only when writable (for us always)

   3) Shared anonymous memory: always

   4) Private anonymous memory: only when writable (for us always)

c) Hugetlbfs: Always


Under Windows: always. On other POSIX besides Linux -- don't know.

-- 
Thanks,

David / dhildenb