Re: [PATCH] mm/slab: Fix undefined init_cache_node_node() for NUMA and !SMP

[Geert Uytterhoeven]

Hi Matthew,

On Wed, Mar 22, 2023 at 5:47 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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.

SH has been using this for a long time.

It's supported. Dave just forgot to update the #ifdef around the
definition of init_cache_node_node() when updating an #ifdef around
a code block that contains one of the callers.

P.S. To me, this discussion reminds me of the old discussion about
     discontigmem without NUMA. Yes, not all systems are PCs with
     contiguous memory on a single fast bus ;-)

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds