Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Wed, 22 Mar 2023 16:46:46 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Geert Uytterhoeven <geert+renesas@glider.be>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Hyeonggon Yoo <42.hyeyoo@gmail.com>,
        John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>,
        linux-mm@kvack.org, linux-sh@vger.kernel.org,
        linux-kernel@vger.kernel.org, Randy Dunlap <rdunlap@infradead.org>
Subject: Re: [PATCH] mm/slab: Fix undefined init_cache_node_node() for NUMA
 and !SMP
Message-ID: <ZBsw9lRbJU4c2wLD@casper.infradead.org>
References: <67261c513706241d479b8b4cf46eb4e6fb0417ba.1679387262.git.geert+renesas@glider.be>
 <ZBneELQuakjva1xa@casper.infradead.org>
 <6320abf6-0898-361b-d5f6-bcc58306f55c@intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <6320abf6-0898-361b-d5f6-bcc58306f55c@intel.com>
Precedence: bulk

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.