Re: [PATCH] arch_numa: remove redundant nodemask clears in numa_init()

[Sang-Heon Jeon]

On Thu, Jun 25, 2026 at 12:40 PM Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, 18 Jun 2026 01:39:19 +0900 Sang-Heon Jeon <ekffu200098@xxxxxxxxx> wrote:
>
> > numa_init() clears numa_nodes_parsed, node_possible_map and
> > node_online_map, then calls numa_memblks_init(), which clears the same
> > nodemasks. Nothing uses them in between.
> >
> > These clears have been redundant since commit 767507654c22 ("arch_numa:
> > switch over to numa_memblks") made numa_init() use numa_memblks_init().
> >
> > No functional change.
> >
> > ...
> >
> > --- a/drivers/base/arch_numa.c
> > +++ b/drivers/base/arch_numa.c
> > @@ -231,10 +231,6 @@ static int __init numa_init(int (*init_func)(void))
> >  {
> >       int ret;
> >
> > -     nodes_clear(numa_nodes_parsed);
> > -     nodes_clear(node_possible_map);
> > -     nodes_clear(node_online_map);
> > -
> >       ret = numa_memblks_init(init_func, /* memblock_force_top_down */ false);
> >       if (ret < 0)
> >               goto out_free_distance;
>
> hm, OK, thanks.
>
>
> A couple of driveby questions:
>
> Are the other nodes_clear() calls are needed - aren't these things
> zeroed when the kernel is loaded?
>

You're talking about nodes_clear() in numa_memblks_init(), right? If
so, I think they're needed.
Because numa_memblks_init() can run more than once during boot, and a
previous call can leave the maps dirty.

More detailed example, If we first try to set up NUMA from DT,
numa_memblks_init(of_numa_init) is called.
of_numa_init() parses the CPU and memory nodes in order, so if parsing
the CPU nodes succeed but the memory nodes fail, the CPU-node bits are
still set in numa_nodes_parsed.
We then fall back to calling numa_memblks_init(dummy_numa_init). Since
dummy_numa_init() only sets node 0, those stale bits could survive
unexpectedly without nodes_clear().

> Also,
>
> #define node_possible_map       node_states[N_POSSIBLE]
>
> ...
>
> nodemask_t node_states[NR_NODE_STATES] __read_mostly = {
>         [N_POSSIBLE] = NODE_MASK_ALL,
>
> why do we carefully initialize node_possible_map at compile-time then
> zero it within __init code?
>

I also can't find why we initialize node_possible_map carefully, at
least for NUMA.
I'm not sure it's safe to remove initialization with UMA. I'll take a
closer look when I have time, and send a patch to remove them if it's
safe to remove.


Best Regards,
Sang-Heon Jeon