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

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Date:   Fri, 25 Sep 2020 17:00:15 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Michal Hocko <mhocko@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.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
 ?

On Fri, Sep 25, 2020 at 04:49:28PM +0200, David Hildenbrand wrote:
> >> 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.

We do care about this, Andrew used to have an old Fedora 9 box or
something like that, that he tourtured many of us with bug reports when
we broke it :)

So watch out, people keep old userspace around for much longer than you
can possibly imagine because they don't like having their use-cases in
userspace change, and we have made the guarantee to them that they _CAN_
trust us to not break things in userspace.

It's a slow age-out, but watch out, you might have to revert things...

good luck!

greg k-h