Re: [PATCH] x86/platform/uv: Use alternate source for socket to node data

[Steve Wahl]

On Mon, Aug 07, 2023 at 09:17:30AM -0500, Steve Wahl wrote:
> When nr_cpus is set to a smaller number than actually present, the
> cpu_to_node() mapping information for unused CPUs is not available to
> build_socket_tables().  This results in an incomplete table and will
> later cause use of a -1 value for some array indexing, and eventual
> kernel page faults.
> 
> Switch to using the __apicid_to_node array, which still contains all
> the information mapping apicids to nodes, even for CPUs disabled with
> a reduced nr_cpus setting.
> 
> Fixes: 8a50c5851927 ("x86/platform/uv: UV support for sub-NUMA clustering")
> Signed-off-by: Steve Wahl <steve.wahl@xxxxxxx>
> ---
>  arch/x86/kernel/apic/x2apic_uv_x.c | 11 +++++------
>  1 file changed, 5 insertions(+), 6 deletions(-)
> 
> This is essentially version 2 of "[PATCH] x86/platform/uv: Abort UV
> initialization when reduced nr_cpus requires it".  However, the title
> was no longer accurate, aborting UV initialization is no longer needed.
> 
> Previous discussion can be found here:
> https://lore.kernel.org/lkml/20230711202618.85562-1-steve.wahl@xxxxxxx/
> 
> diff --git a/arch/x86/kernel/apic/x2apic_uv_x.c b/arch/x86/kernel/apic/x2apic_uv_x.c
> index d9384d5b4b8e..35acc95c6dd5 100644
> --- a/arch/x86/kernel/apic/x2apic_uv_x.c
> +++ b/arch/x86/kernel/apic/x2apic_uv_x.c
> @@ -1571,7 +1571,7 @@ static void __init build_socket_tables(void)
>  {
>  	struct uv_gam_range_entry *gre = uv_gre_table;
>  	int nums, numn, nump;
> -	int cpu, i, lnid;
> +	int i, lnid, apicid;
>  	int minsock = _min_socket;
>  	int maxsock = _max_socket;
>  	int minpnode = _min_pnode;
> @@ -1622,15 +1622,14 @@ static void __init build_socket_tables(void)
>  
>  	/* Set socket -> node values: */
>  	lnid = NUMA_NO_NODE;
> -	for_each_possible_cpu(cpu) {
> -		int nid = cpu_to_node(cpu);
> -		int apicid, sockid;
> +	for (apicid = 0; apicid < ARRAY_SIZE(__apicid_to_node); apicid++) {
> +		int nid = __apicid_to_node[apicid];
> +		int sockid;
>  
> -		if (lnid == nid)
> +		if ((nid == NUMA_NO_NODE) || (lnid == nid))
>  			continue;
>  		lnid = nid;
>  
> -		apicid = per_cpu(x86_cpu_to_apicid, cpu);
>  		sockid = apicid >> uv_cpuid.socketid_shift;
>  
>  		if (_socket_to_node[sockid - minsock] == SOCK_EMPTY)
> -- 
> 2.26.2
> 

Gentle ping on this patch?  Thanks.

--> Steve Wahl

-- 
Steve Wahl, Hewlett Packard Enterprise