Re: Ways to deprecate /sys/devices/system/memory/memoryX/phys_device ?

From: David Hildenbrand <david@redhat.com>
To: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Cc: "Michal Hocko" <mhocko@kernel.org>,
	"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
	"Greg KH" <gregkh@linuxfoundation.org>,
	"Jan Höppner" <hoeppner@linux.ibm.com>,
	"Heiko Carstens" <hca@linux.ibm.com>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	linux-api@vger.kernel.org,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: Ways to deprecate /sys/devices/system/memory/memoryX/phys_device ?
Date: Fri, 25 Sep 2020 16:49:28 +0200	[thread overview]
Message-ID: <a7d02345-2195-3092-a368-ca3209e2c93e@redhat.com> (raw)
In-Reply-To: <20200922155611.379373f7@thinkpad>

>> There were once RFC patches to make use of it in ACPI, but it could be
>> solved using different interfaces [1].
>>
>>
>> While I'd love to rip it out completely, I think it would break old
>> lsmem/chmem completely - and I assume that's not acceptable. I was
>> wondering what would be considered safe to do now/in the future:
>>
>> 1. Make it always return 0 (just as if "sclp.rzm" would be set to 0 on
>> s390x). This will make old lsmem/chmem behave differently after
>> switching to a new kernel, like if sclp.rzm would not be set by HW -
>> AFAIU, it will assume all memory is in a single memory increment. Do we
>> care?
> 
> No, at least not until that kernel change would be backported to some
> old distribution level where we still use lsmem/chmem from s390-tools.
> Given that this is just some clean-up w/o any functional benefit, and
> hopefully w/o any negative impact, I think we can safely assume that no
> distributor will do that "just for fun".
> 
> Even if there would be good reasons for backports, then I guess we also
> have good reasons for backporting / switching to the util-linux version
> of lsmem / chmem for such distribution levels. Alternatively, adjust the
> s390-tools lsmem / chmem there.
> 
> But I would rather "rip it out completely" than just return 0. You'd
> need some lsmem / chmem changes anyway, at least in case this would
> ever be backported.

Thanks for your input Gerald.

So unless people would be running shiny new kernels on older
distributions it shouldn't be a problem (and I don't think we care too
much about something like that). I don't expect something like that to
get backported - there is absolutely no reason to do so IMHO.

> 
>> 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)

-- 
Thanks,

David / dhildenb