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

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Message-ID: <0175453a-7969-f482-a228-34c5840fe0e0@intel.com>
Date:   Thu, 10 Sep 2020 13:57:33 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     David Hildenbrand <david@...hat.com>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Michal Hocko <mhocko@...nel.org>,
        "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
 ?

On 9/10/20 3:20 AM, David Hildenbrand wrote:
> 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?
> 2. Restrict it to s390x only. It always returned 0 on other
> architectures, I was not able to find any user.

By "restrict it", do you mean just remove the sysfs file on everything
other than s390x?  That seems like a good idea, especially if we don't
have any users.  That, plus boot option or something to reenable it
would be nice if someone trips over it disappearing.

If there is a user, we stand a chance of finding them because they'll
hopefully get a good error message.  Worst case, an strace will show an
-ENOENT and should be pretty easy to track down.