Re: [PATCH] x86/mm: drop 4MB restriction on minimal NUMA node size

[Mario Casquero]

This patch has been successfully tested by QE. Start a VM with two
NUMA nodes, one of them with less than 2M of memory. Check there is no
kernel panic and the VM boots up smoothly.
Tested-by: Mario Casquero <mcasquer@xxxxxxxxxx>

BR,
Mario




On Tue, Oct 17, 2023 at 8:24 AM Mike Rapoport <rppt@xxxxxxxxxx> wrote:
>
> From: "Mike Rapoport (IBM)" <rppt@xxxxxxxxxx>
>
> Qi Zheng reports crashes in a production environment and provides a
> simplified example as a reproducer:
>
>   For example, if we use qemu to start a two NUMA node kernel,
>   one of the nodes has 2M memory (less than NODE_MIN_SIZE),
>   and the other node has 2G, then we will encounter the
>   following panic:
>
>   [    0.149844] BUG: kernel NULL pointer dereference, address: 0000000000000000
>   [    0.150783] #PF: supervisor write access in kernel mode
>   [    0.151488] #PF: error_code(0x0002) - not-present page
>   <...>
>   [    0.156056] RIP: 0010:_raw_spin_lock_irqsave+0x22/0x40
>   <...>
>   [    0.169781] Call Trace:
>   [    0.170159]  <TASK>
>   [    0.170448]  deactivate_slab+0x187/0x3c0
>   [    0.171031]  ? bootstrap+0x1b/0x10e
>   [    0.171559]  ? preempt_count_sub+0x9/0xa0
>   [    0.172145]  ? kmem_cache_alloc+0x12c/0x440
>   [    0.172735]  ? bootstrap+0x1b/0x10e
>   [    0.173236]  bootstrap+0x6b/0x10e
>   [    0.173720]  kmem_cache_init+0x10a/0x188
>   [    0.174240]  start_kernel+0x415/0x6ac
>   [    0.174738]  secondary_startup_64_no_verify+0xe0/0xeb
>   [    0.175417]  </TASK>
>   [    0.175713] Modules linked in:
>   [    0.176117] CR2: 0000000000000000
>
> The crashes happen because of inconsistency between nodemask that has
> nodes with less than 4MB as memoryless and the actual memory fed into
> core mm.
>
> The commit 9391a3f9c7f1 ("[PATCH] x86_64: Clear more state when ignoring
> empty node in SRAT parsing") that introduced minimal size of a NUMA node
> does not explain why a node size cannot be less than 4MB and what boot
> failures this restriction might fix.
>
> Since then a lot has changed and core mm won't confuse badly about small
> node sizes.
>
> Drop the limitation for the minimal node size.
>
> Reported-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> Signed-off-by: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> Link: https://lore.kernel.org/all/20230212110305.93670-1-zhengqi.arch@xxxxxxxxxxxxx/
> ---
>  arch/x86/include/asm/numa.h | 7 -------
>  arch/x86/mm/numa.c          | 7 -------
>  2 files changed, 14 deletions(-)
>
> diff --git a/arch/x86/include/asm/numa.h b/arch/x86/include/asm/numa.h
> index e3bae2b60a0d..ef2844d69173 100644
> --- a/arch/x86/include/asm/numa.h
> +++ b/arch/x86/include/asm/numa.h
> @@ -12,13 +12,6 @@
>
>  #define NR_NODE_MEMBLKS                (MAX_NUMNODES*2)
>
> -/*
> - * Too small node sizes may confuse the VM badly. Usually they
> - * result from BIOS bugs. So dont recognize nodes as standalone
> - * NUMA entities that have less than this amount of RAM listed:
> - */
> -#define NODE_MIN_SIZE (4*1024*1024)
> -
>  extern int numa_off;
>
>  /*
> diff --git a/arch/x86/mm/numa.c b/arch/x86/mm/numa.c
> index 2aadb2019b4f..55e3d895f15c 100644
> --- a/arch/x86/mm/numa.c
> +++ b/arch/x86/mm/numa.c
> @@ -601,13 +601,6 @@ static int __init numa_register_memblks(struct numa_meminfo *mi)
>                 if (start >= end)
>                         continue;
>
> -               /*
> -                * Don't confuse VM with a node that doesn't have the
> -                * minimum amount of memory:
> -                */
> -               if (end && (end - start) < NODE_MIN_SIZE)
> -                       continue;
> -
>                 alloc_node_data(nid);
>         }
>
>
> base-commit: 94f6f0550c625fab1f373bb86a6669b45e9748b3
> --
> 2.39.2
>