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Re: [PATCH v6 14/15] qmp: Include "reserve" property of memory backends
From: |
Markus Armbruster |
Subject: |
Re: [PATCH v6 14/15] qmp: Include "reserve" property of memory backends |
Date: |
Fri, 23 Apr 2021 16:55:18 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) |
David Hildenbrand <david@redhat.com> writes:
> On 23.04.21 14:29, David Hildenbrand wrote:
>> On 23.04.21 14:13, Markus Armbruster wrote:
>>> David Hildenbrand <david@redhat.com> writes:
>>>
>>>> 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
>
> Actually, it's a bit simpler in our case (writable mappings). The more
> complex part is the MAP_NORESERVE handling.
>
> a) !Hugetlbfs
> 1) Shared mappings on files: never reserve swap space
>
> 2) Other mappings: always reserve swap space
>
> b) Hugetlbfs: Always reserve huge pages.
>
>>>>
>>>>
>>>> Under Windows: always. On other POSIX besides Linux -- don't know.
>>>
>>> Would working some of this into the doc comment help users of the
>>> interface? Up to you.
>>>
>> I'll give it a thought. Most people will only care about explicitly
>> disabling it, where we'll bail out if that doesn't work. Otherwise, they
>> just use the OS default (== reserve if supported/applicable/not
>> explicitly disabled).
>
> I think something like the following might make sense:
>
> # @reserve: whether swap space (or huge pages) was reserved if applicable.
> # This corresponds to the user configuration and not the actual
> # behavior implemented in the OS to perform a reservation;
> # For example, Linux will never reserve swap space for shared
> # file mappings. (since 6.1)
>
> Thanks!
Good balance between too little and too much detail.
Reviewed-by: Markus Armbruster <armbru@redhat.com>
- Re: [PATCH v6 13/15] hmp: Print "share" property of memory backends with "info memdev", (continued)
- [PATCH v6 14/15] qmp: Include "reserve" property of memory backends, David Hildenbrand, 2021/04/21
- Re: [PATCH v6 14/15] qmp: Include "reserve" property of memory backends, Eduardo Habkost, 2021/04/21
- Re: [PATCH v6 14/15] qmp: Include "reserve" property of memory backends, Markus Armbruster, 2021/04/23
- Re: [PATCH v6 14/15] qmp: Include "reserve" property of memory backends, David Hildenbrand, 2021/04/23
- Re: [PATCH v6 14/15] qmp: Include "reserve" property of memory backends, Markus Armbruster, 2021/04/23
- Re: [PATCH v6 14/15] qmp: Include "reserve" property of memory backends, David Hildenbrand, 2021/04/23
- Re: [PATCH v6 14/15] qmp: Include "reserve" property of memory backends, Markus Armbruster, 2021/04/23
- Re: [PATCH v6 14/15] qmp: Include "reserve" property of memory backends, David Hildenbrand, 2021/04/23
- Re: [PATCH v6 14/15] qmp: Include "reserve" property of memory backends, David Hildenbrand, 2021/04/23
- Re: [PATCH v6 14/15] qmp: Include "reserve" property of memory backends,
Markus Armbruster <=
- Re: [PATCH v6 14/15] qmp: Include "reserve" property of memory backends, David Hildenbrand, 2021/04/23
[PATCH v6 15/15] hmp: Print "reserve" property of memory backends with "info memdev", David Hildenbrand, 2021/04/21
Re: [PATCH v6 00/15] RAM_NORESERVE, MAP_NORESERVE and hostmem "reserve" property, Eduardo Habkost, 2021/04/21