linux-kernel - Re: [PATCH] mm/slab: Fix undefined init_cache_node

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Date:   Wed, 22 Mar 2023 09:49:48 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Geert Uytterhoeven <geert+renesas@...der.be>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        linux-mm@...ck.org, linux-sh@...r.kernel.org,
        linux-kernel@...r.kernel.org, Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH] mm/slab: Fix undefined init_cache_node_node() for NUMA
 and !SMP

On 3/22/23 09:46, Matthew Wilcox wrote:
> On Wed, Mar 22, 2023 at 09:16:55AM -0700, Dave Hansen wrote:
>> On 3/21/23 09:40, Matthew Wilcox wrote:
>>> On Tue, Mar 21, 2023 at 09:30:59AM +0100, Geert Uytterhoeven wrote:
>>>> -#if (defined(CONFIG_NUMA) && defined(CONFIG_MEMORY_HOTPLUG)) || defined(CONFIG_SMP)
>>>> +#if defined(CONFIG_NUMA) || defined(CONFIG_SMP)
>>> I'm amused by the thought of CONFIG_NUMA without CONFIG_SMP.
>>> Is it possible to have one node with memory and a single CPU, then
>>> another node with memory and no CPU?
>> It's _possible_ for sure, just unlikely.  The most likely place these
>> days is probably a teensy tiny VM that just happens to have some
>> performance-differentiated memory exposed to it for some reason.  Maybe
>> it's got a slice of slow PMEM or fast High-Bandwidth memory for whatever
>> reason.
> Right, you can construct such a system, but do we support the CONFIG
> options of NUMA enabled and SMP disabled?  It seems so niche that we
> shouldn't be spending time testing that combination.

On x86 we don't:

> config NUMA
>         bool "NUMA Memory Allocation and Scheduler Support"
>         depends on SMP
>         depends on X86_64 || (X86_32 && HIGHMEM64G && X86_BIGSMP)

... which I think is fine.  I totally agree that NUMA without SMP is too
niche to care about.  Heck, !SMP is almost too niche to care about these
days.