linux-kernel - Re: [PATCH RFC] drivers/base/memory.c: indicate all memory blocks as removable

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Message-ID: <20200127132552.GG1183@dhcp22.suse.cz>
Date:   Mon, 27 Jan 2020 14:25:52 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     "Fontenot, Nathan" <ndfont@...il.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Dan Williams <dan.j.williams@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        powerpc-utils-devel@...glegroups.com, util-linux@...r.kernel.org,
        Badari Pulavarty <pbadari@...ibm.com>,
        Robert Jennings <rcj@...ux.vnet.ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Karel Zak <kzak@...hat.com>
Subject: Re: [PATCH RFC] drivers/base/memory.c: indicate all memory blocks as
 removable

On Mon 27-01-20 10:33:55, David Hildenbrand wrote:
> On 27.01.20 10:23, Michal Hocko wrote:
> > On Fri 24-01-20 13:10:22, Fontenot, Nathan wrote:
> >> It's been awhile since I've looked at the powerpc-utils drmgr command and
> >> pseries DLPAR code but a quick scan makes and it appears that it hasn't changed
> >> too much. Given that, some thoughts.
> >>
> >> The sysfs 'removable' file was a great help when memory DLPAR was driven
> >> from userspace in the powerpc-utils drmgr command. Having this check did improve
> >> performance though I can't point to any numbers.
> > 
> > Do you still have an access to the HW to give it a try?
> > 
> >> Currently, memory DLPAR is done completely in the kernel. The request is
> >> initiated from drmgr writing to /sys/kernel/dlpar (for pHyp partitions)
> >> or from a hotplug interrupt (for guests). I don't believe the 'removable'
> >> sysfs file is used in either of these paths by drmgr. The only time it is
> >> used is on older kernels that do not support in-kernel memory DLPAR.
> >>
> >> Given this, I don't think removing the 'removable' sysfs file would cause any
> >> issues for the drmgr command. The only scenario I can think of is using an old
> >> version of drmgr that does not support in-kernel memory DLPAR on a new kernel
> >> where the 'removable' sysfs file has been removed. This doesn't seem likely
> >> though and drmgr could be updated to detect this.
> > 
> > Thanks for the information!
> > 
> 
> (weird, I never received the mail from Nathan - mail deliver issues
> brighten my Mondays :) )
> 
> Thanks for the information! Looks like powerpc indeed can live without
> the interface (old userspace on shiny new kernel would in the worst case
> simply be slower).
> 
> Of course, the alternative to returning always "removable" would be to
> drop the attribute completely. So, if the "removable" attribute is not
> present
> 
> - powerpc-utils will fallback to "removable"
> - lsmem will fallback to "not removable". Could be because it assumes
>   "old kernel with lacking offlining capability".
> 
> I don't know how likely it is that this could break custom scripts that
> used the returned value for any purpose (e.g., use it as an indicator if
> memory offlining is supported at all etc.).

Our long term tradition with user visible knobs is to keep them in place
and simply fake the answer. This seems to be a safer option and less
likely to lead to failures.
-- 
Michal Hocko
SUSE Labs