linux-kernel - Re: [RFC PATCH 1/4] mm/memory_hotplug: Add interface for runtime (de)configuration of memory

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Message-ID: <63967c9a-0ae7-42c1-9a2e-31213ad1fa90@redhat.com>
Date: Wed, 21 May 2025 16:25:32 +0200
From: David Hildenbrand <david@...hat.com>
To: Heiko Carstens <hca@...ux.ibm.com>
Cc: Sumanth Korikkar <sumanthk@...ux.ibm.com>, linux-mm <linux-mm@...ck.org>,
 Andrew Morton <akpm@...ux-foundation.org>, Oscar Salvador
 <osalvador@...e.de>, Gerald Schaefer <gerald.schaefer@...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 21.05.25 16:21, Heiko Carstens wrote:
> On Wed, May 21, 2025 at 02:33:42PM +0200, David Hildenbrand wrote:
>> On 21.05.25 12:34, Sumanth Korikkar wrote:
>>> As you pointed out, how about having something similar to
>>> 73954d379efd ("dax: add a sysfs knob to control memmap_on_memory behavior")
>>
>> Right. But here, the use case is usually (a) to add a gigantic amount of
>> memory using add_memory(), not small blocks like on s390x (b) consume the
>> memmap from (slow) special-purpose memory as well.
>>
>> Regarding (a), the memmap could be so big that add_memory() might never
>> really work (not just because of some temporary low-memory situation).
> 
> What is "big"? Worst case for s390 with existing machines would be an
> increment size (aka memory block size) of 64GB.
Oh! I was assuming the increment size would always be around 256MiB or so.

In that case, it can make sense to have this, yes!

-- 
Cheers,

David / dhildenb