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

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Date:   Wed, 31 Jul 2019 16:04:10 +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 15:42, David Hildenbrand wrote:
> On 31.07.19 15:25, Michal Hocko wrote:
>> On Wed 31-07-19 15:12:12, David Hildenbrand wrote:
>>> On 31.07.19 14:43, Michal Hocko wrote:
>>>> On Wed 31-07-19 14:22:13, David Hildenbrand wrote:
>>>>> Each memory block spans the same amount of sections/pages/bytes. The size
>>>>> is determined before the first memory block is created. No need to store
>>>>> what we can easily calculate - and the calculations even look simpler now.
>>>>
>>>> While this cleanup helps a bit, I am not sure this is really worth
>>>> bothering. I guess we can agree when I say that the memblock interface
>>>> is suboptimal (to put it mildly).  Shouldn't we strive for making it
>>>> a real hotplug API in the future? What do I mean by that? Why should
>>>> be any memblock fixed in size? Shouldn't we have use hotplugable units
>>>> instead (aka pfn range that userspace can work with sensibly)? Do we
>>>> know of any existing userspace that would depend on the current single
>>>> section res. 2GB sized memblocks?
>>>
>>> Short story: It is already ABI (e.g.,
>>> /sys/devices/system/memory/block_size_bytes) - around since 2005 (!) -
>>> since we had memory block devices.
>>>
>>> I suspect that it is mainly manually used. But I might be wrong.
>>
>> Any pointer to the real userspace depending on it? Most usecases I am
>> aware of rely on udev events and either onlining or offlining the memory
>> in the handler.
> 
> Yes, that's also what I know - onlining and triggering kexec().
> 
> On s390x, admins online sub-increments to selectively add memory to a VM
> - but we could still emulate that by adding memory for that use case in
> the kernel in the current granularity. See
> 
> https://books.google.de/books?id=afq4CgAAQBAJ&pg=PA117&lpg=PA117&dq=/sys/devices/system/memory/block_size_bytes&source=bl&ots=iYk_vW5O4G&sig=ACfU3U0s-O-SOVaQO-7HpKO5Hj866w9Pxw&hl=de&sa=X&ved=2ahUKEwjOjPqIot_jAhVPfZoKHcxpAqcQ6AEwB3oECAgQAQ#v=onepage&q=%2Fsys%2Fdevices%2Fsystem%2Fmemory%2Fblock_size_bytes&f=false
> 
>>
>> 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

FWIW, powerpc-utils also uses the "removable" property - which means
we're also stuck with that unfortunately. :(

-- 

Thanks,

David / dhildenb