linux-kernel - Re: Ways to deprecate /sys/devices/system/memory/memoryX/phys

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Date:   Fri, 25 Sep 2020 17:47:18 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Jan Höppner <hoeppner@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        linux-api@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Ways to deprecate /sys/devices/system/memory/memoryX/phys_device
 ?

>>>> 2. Restrict it to s390x only. It always returned 0 on other
>>>> architectures, I was not able to find any user.
>>>>
>>>> I think 2 should be safe to do (never used on other archs). I do wonder
>>>> what the feelings are about 1.
>>>
>>> Please don't add any s390-specific workarounds here, that does not
>>> really sound like a clean-up, rather the opposite.
>>
>> People seem to have different opinions here. I'm happy as long as we can
>> get rid of it (either now, or in the future with a new model).
>>
>>>
>>> That being said, I do not really see the benefit of this change at
>>> all. As Michal mentioned, there really should be some more fundamental
>>> change. And from the rest of this thread, it also seems that phys_device
>>> usage might not be the biggest issue here.
>>>
>>
>> As I already expressed, I am more of a friend of small, incremental
>> changes than having a single big world switch where everything will be
>> shiny and perfect.
>>
>> (Deprecating it now - in any way - stops any new users from appearing -
>> both, in the kernel and from user space - eventually making the big
>> world switch later a little easier because there is one thing less that
>> vanished)
>
> Realistically people do not care about deprecation all that much. They
> simply use whatever they can find or somebody will show them. Really,
> deprecation has never really worked. The only thing that worked was to
> remove the functionality and then wait for somebody to complain and
> revert or somehow allow the functionality without necessity to alter the
> userspace.

Mainframe people are usually ... more conservative (well, they focus on
stability and pay a lot of money for that - including HW). :)

What they would lose here is s390x lsmem/chmem functionality, used to
manage standby memory (under LPAR and z/VM, if enabled) - with the old
tools. I have the feeling that this would be acceptable (I never had
access to an LPAR that allowed for it ...), but yeah, you never now.

> 
> As much as I would like to remove as much crud as possible I strongly
> suspect that the existing hotplug interface is just a lost case and it
> doesn't make for the best used time to put a lip stick on a pig. Even if
> we remove this particular interface we are not going to get rid of a lot
> of code or we won't gain any more sensible semantic, right?
> 

Excluding some documentation

 drivers/base/memory.c        | 29 -----------------------------
 drivers/s390/char/sclp_cmd.c |  7 -------
 include/linux/memory.h       |  2 --
 3 files changed, 38 deletions(-)

Seems like this is the only way to deprecate. (I mean I can add comments
in the code, but as you say, doesn't stop new user space users from
showing up)

-- 
Thanks,

David / dhildenb