Message-ID: <1b9285ba-4118-4572-9392-42ec6ba6728c@redhat.com>
Date: Fri, 20 Dec 2024 16:53:01 +0100
From: David Hildenbrand <david@...hat.com>
To: Sumanth Korikkar <sumanthk@...ux.ibm.com>
Cc: linux-mm <linux-mm@...ck.org>, Andrew Morton <akpm@...ux-foundation.org>,
 Oscar Salvador <osalvador@...e.de>,
 Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
 Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
 Alexander Gordeev <agordeev@...ux.ibm.com>,
 linux-s390 <linux-s390@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/4] mm/memory_hotplug: Add interface for runtime
 (de)configuration of memory

On 03.12.24 15:33, Sumanth Korikkar wrote:
> On Mon, Dec 02, 2024 at 05:55:19PM +0100, David Hildenbrand wrote:
>> Hi!
>>
>> Not completely what I had in mind, especially not that we need something
>> that generic without any indication of ranges :)
>>
>> In general, the flow is as follows:
>>
>> 1) Driver detects memory and adds it
>> 2) Something auto-onlines that memory (e.g., udev rule)
>>
>> For dax/kmem, 1) can be controlled using devdax, and usually it also tries
>> to take care of 2).
>>
>> s390x standby storage really is the weird thing here, because it does 1) and
>> doesn't want 2). It shouldn't do 1) until a user wants to make use of
>> standby memory.
> 
> Hi David,

Hi,

sorry for the late reply. Cleaning up (some of) my inbox before 
Christmas, and I realized I skipped this mail.

> 
> The current rfc design doesnt do 1) until user initiates it.
> 
> The current rfc design considers the fact that there cannot be memory
> holes, when there is a availability of standby memory. (which holds true
> for both lpars and zvms)
> 
> With number of online and standby memory ranges count
> (max_configurable), prototype lsmem/chmem could determine memory ranges
> which are not yet configured
> i.e. (configurable_memory = max_configurable - online ranges from sysfs
> /sys/devices/system/memory/memory*).
> 
> Example prototype implementation of lsmem/chmem looks like:
> ./lsmem -o RANGE,SIZE,STATE,BLOCK,ALTMAP
> RANGE                                 SIZE        STATE  BLOCK ALTMAP
> 0x0000000000000000-0x00000002ffffffff  12G       online   0-95      0
> 0x0000000300000000-0x00000003ffffffff   4G deconfigured 96-127      -
> 
> # Configure range with altmap
> ./chmem -c 0x0000000300000000-0x00000003ffffffff -a
> ./lsmem -o RANGE,SIZE,STATE,BLOCK,ALTMAP
> RANGE                                 SIZE   STATE  BLOCK ALTMAP
> 0x0000000000000000-0x00000002ffffffff  12G  online   0-95      0
> 0x0000000300000000-0x00000003ffffffff   4G offline 96-127      1
> 
> 
> # Online range
> ./chmem -e 0x0000000300000000-0x00000003ffffffff &&
> ./lsmem -o RANGE,SIZE,STATE,BLOCK,ALTMAP
> RANGE                                 SIZE  STATE  BLOCK ALTMAP
> 0x0000000000000000-0x00000002ffffffff  12G online   0-95      0
> 0x0000000300000000-0x00000003ffffffff   4G online 96-127      1
> 
> Memory block size:       128M
> Total online memory:      16G
> Total offline memory:      0B
> Total deconfigured:        0B
> 
> # offline range
> ./chmem -d 0x0000000300000000-0x00000003ffffffff &&
> ./lsmem -o RANGE,SIZE,STATE,BLOCK,ALTMAP
> RANGE                                 SIZE   STATE  BLOCK ALTMAP
> 0x0000000000000000-0x00000002ffffffff  12G  online   0-95      0
> 0x0000000300000000-0x00000003ffffffff   4G offline 96-127      1
> 
> Memory block size:       128M
> Total online memory:      12G
> Total offline memory:      4G
> Total deconfigured:        0B
> 
> # Defconfigure range.
> ./chmem -g 0x0000000300000000-0x00000003ffffffff &&
> ./lsmem -o RANGE,SIZE,STATE,BLOCK,ALTMAP
> RANGE                                 SIZE        STATE  BLOCK ALTMAP
> 0x0000000000000000-0x00000002ffffffff  12G       online   0-95      0
> 0x0000000300000000-0x00000003ffffffff   4G deconfigured 96-127      -
> 
> Memory block size:       128M
> Total online memory:      12G
> Total offline memory:      0B
> Total deconfigured:        4G

Maybe "standby memory" might make it clearer. The concept is s390x 
specific, and it will likely stay s390x specific.

I like the idea (frontend/tool interface), all we need is a way for 
these commands to detect ranges and turn them from standby into usable 
memory.

> 
> The user can still determine the available memory ranges and make them
> configurable using tools like lsmem or chmem with this approach atleast
> on s390 with this approach.
> 
>> My thinking was that s390x would expose the standby memory ranges somewhere
>> arch specific in sysfs. From there, one could simply trigger the adding
>> (maybe specifying e.g, memmap_on_memory) of selected ranges.
> 
> As far as I understand, sysfs interface limits the size of the buffer
> used in show() to 4kb. 

sysfs want usually "one value per file".

> When there are huge number of standby memory
> ranges, wouldnt it be an issue to display everything in one attribute?

I was rather wondering about a syfs directory structure that exposes 
this information.

For example, in the granularity of storage increments we can enable/disable.

In general, it could be a similar structure as 
/sys/devices/system/memory/ (one director = one standby storage 
increment we can enable/disable?), but residing on the s390x specific 
sysfs area. Or any other way to express ranges that can be 
enabled/disabled as one unit.

I'm not sure if extending /sys/devices/system/memory/ itself would be a 
good idea, though. It all is very s390x specific.

> 
> Or use sysfs binary attributes to overcome the limitation?
> 
> Please correct me, If I am wrong.
> 
> Questions:
> 1. If we go ahead with this sysfs interface approach to list all standby
> memory ranges, could the list be made available via
> /sys/devices/system/memory/configurable_memlist?  This could be helpful,
> as /sys/devices/system/memory/configure_memory performs architecture
> independent checks and could also be useful for other architectures in
> the future.

See above, I think we want this s390x specific.

> 
> 2. Whether the new interface should also be compatible with lsmem/chmem?

Yes, likely we should allow them to query-configure this s390x specific 
thing.

> 
> 3. OR can we have a s390 specific path (eg:
> /sys/firmware/memory/standy_range) to list all standby memory range
> which are in deconfigured state and also use the current design
> (max_configurable) to make it easier for lsmem/chmem tool to detect
> these standby memory ranges?

Ah, there it is, yes!

> 
>> To disable standby memory, one would first offline the memory to then
>> trigger removal using the arch specific interface. It is very similar to
>> dax/kmem's way of handling offline+removal.
> 
> ok
> 
>> Now I wonder if dax/kmem could be (ab)used on s390x for standby storage.
>> Likely a simple sysfs interface could be easier to implement.
> 
> I havent checked dax/kmem in detail yet. I will look into it.

Probably it's not 100% what you want to achieve, just to give you an 
example how similar (but different) technologies have solved this problem.

-- 
Cheers,

David / dhildenb

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