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

[Mike Rapoport]

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