On Wed 15-02-23 11:30:19, Mike Rapoport wrote:
[...]
> >From b670120bcacd3fe34a40d7179c70ca2ab69279e0 Mon Sep 17 00:00:00 2001
> From: "Mike Rapoport (IBM)" <rppt@xxxxxxxxxx>
> Date: Wed, 15 Feb 2023 11:12:18 +0200
> Subject: [PATCH] x86/mm: drop 4MB restriction on minimal NUMA node size
>
> 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.
>
> Link: https://lore.kernel.org/all/20230212110305.93670-1-zhengqi.arch@xxxxxxxxxxxxx/
> Signed-off-by: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>
Yes, I would start with this. If the original reasoning to have a limit
still exists then we would need to have a closer look but right now I
would much rather drop this unexplained subtlety. If we hit the issue we
would get a more specific description at least.
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Thanks!
> ---
> 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);
> }
>
> --
> 2.35.1
>
>
> > --
> > Michal Hocko
> > SUSE Labs
>
> --
> Sincerely yours,
> Mike.
--
Michal Hocko
SUSE Labs
