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

From: "Fontenot, Nathan" <ndfont@gmail.com>
To: David Hildenbrand <david@redhat.com>, Michal Hocko <mhocko@kernel.org>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Dan Williams <dan.j.williams@intel.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	powerpc-utils-devel@googlegroups.com, util-linux@vger.kernel.org,
	Badari Pulavarty <pbadari@us.ibm.com>,
	Robert Jennings <rcj@linux.vnet.ibm.com>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	Karel Zak <kzak@redhat.com>
Subject: Re: [PATCH RFC] drivers/base/memory.c: indicate all memory blocks as removable
Date: Mon, 27 Jan 2020 09:23:55 -0600
Message-ID: <c5059edf-1fdd-1203-4717-2512e8c364be@gmail.com> (raw)
In-Reply-To: <d2b3adaa-bd4d-d105-f8ab-0301409bbd5f@redhat.com>

On 1/27/2020 3:33 AM, 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?

No, I no longer have access to Power hardware.

-Nathan

>>
>>> 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.).
> 