[PATCH v2] x86/mm: Drop 4MB restriction on minimal NUMA node memory size

[Ingo Molnar]

* 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.

Presumably the core MM got fixed too to not just crash, but provide some 
sort of warning?

> 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.

Core MM won't get confused ... other than by the above weird Qemu topology, 
to which it responds with a ... NULL pointer dereference?

Seems quite close to the literal definition of 'get confused badly' to me, 
and doesn't give me the warm fuzzy feeling that giving the core MM even 
*more* weird topologies is super safe ... :-/

> Drop the limitation for the minimal node size.

While I agree with dropping the limitation, and I agree that 9391a3f9c7f1 
should have provided more of a justification, I believe a core MM fix is in 
order as well, for it to not crash. [ If it's fixed upstream already, 
please reference the relevant commit ID. ]

Also, the changelog spelling & general presentation were quite low quality 
- I've fixed it up a bit below, please carry this version going forward. 
Please spell-check your patches before sending out Nth versions of it, 
maybe maintainers are skipping them for a reason!

Thanks,

	Ingo

=================>
From: "Mike Rapoport (IBM)" <rppt@xxxxxxxxxx>
Date: Tue, 17 Oct 2023 09:22:15 +0300
Subject: [PATCH] x86/mm: Drop 4MB restriction on minimal NUMA node memory size

Qi Zheng reported crashes in a production environment and provided 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:
 |
 |    BUG: kernel NULL pointer dereference, address: 0000000000000000
 |    <...>
 |    RIP: 0010:_raw_spin_lock_irqsave+0x22/0x40
 |    <...>
 |    Call Trace:
 |      <TASK>
 |      deactivate_slab()
 |      bootstrap()
 |      kmem_cache_init()
 |      start_kernel()
 |      secondary_startup_64_no_verify()

The crashes happen because of inconsistency between the nodemask that
has nodes with less than 4MB as memoryless, and the actual memory fed
into the 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.

In the 17 years since then a lot has changed and core mm won't get
confused about small node sizes.

Drop the limitation for the minimal node size.

[ mingo: Improved changelog clarity. ]

Reported-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
Signed-off-by: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>
Not-Yet-Signed-off-by: Ingo Molnar <mingo@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 c01c5506fd4a..aa39d678fe81 100644
--- a/arch/x86/mm/numa.c
+++ b/arch/x86/mm/numa.c
@@ -602,13 +602,6 @@ 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);
 	}