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

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Message-ID: <20250521142149.11483C95-hca@linux.ibm.com>
Date: Wed, 21 May 2025 16:21:49 +0200
From: Heiko Carstens <hca@...ux.ibm.com>
To: David Hildenbrand <david@...hat.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 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. So more than 1GB for
memmap plus pages tables, etc would be required.