linux-kernel - Re: [PATCH v1] drivers/base/memory.c: Don't store end_section

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Message-ID: <c92a4d6f-b0f2-e080-5157-b90ab61a8c49@redhat.com>
Date:   Wed, 31 Jul 2019 16:21:46 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH v1] drivers/base/memory.c: Don't store end_section_nr in
 memory blocks

On 31.07.19 16:14, Michal Hocko wrote:
> On Wed 31-07-19 15:42:53, David Hildenbrand wrote:
>> On 31.07.19 15:25, Michal Hocko wrote:
> [...]
>>> I know we have documented this as an ABI and it is really _sad_ that
>>> this ABI didn't get through normal scrutiny any user visible interface
>>> should go through but these are sins of the past...
>>
>> A quick google search indicates that
>>
>> Kata containers queries the block size:
>> https://github.com/kata-containers/runtime/issues/796
>>
>> Powerpc userspace queries it:
>> https://groups.google.com/forum/#!msg/powerpc-utils-devel/dKjZCqpTxus/AwkstV2ABwAJ
>>
>> I can imagine that ppc dynamic memory onlines only pieces of added
>> memory - DIMMs AFAIK (haven't looked at the details).
>>
>> There might be more users.
> 
> Thanks! I suspect most of them are just using the information because
> they do not have anything better.

powerpc-utils actually seem to use the fine-grained API to dynamically
manage memory assignment to the VM.

> 
> Thinking about it some more, I believe that we can reasonably provide
> both APIs controlable by a command line parameter for backwards
> compatibility. It is the hotplug code to control sysfs APIs.  E.g.
> create one sysfs entry per add_memory_resource for the new semantic.

Yeah, but the real question is: who needs it. I can only think about
some DIMM scenarios (some, not all). I would be interested in more use
cases. Of course, to provide and maintain two APIs we need a good reason.

(one sysfs per add_memory_resource() won't cover all DIMMs completely as
far as I remember - I might be wrong, I remember there could be a
sequence of add_memory(). Also, some DIMMs might actually overlap with
memory indicated during boot - complicated stuff)

> 
> It is some time since I've checked the ACPI side of the matter but that
> code shouldn't really depend on a particular size of the memblock
> either when trigerring udev events. I might be wrong here of course.

It only has to respect the alignment/size restriction when calling
add_memory() right now. That would map to a "minimum block size"

-- 

Thanks,

David / dhildenb