slab: fix barn NULL pointer dereference on memoryless nodes

Phil reported a boot failure once sheaves become used in commits
59faa4da7cd4 ("maple_tree: use percpu sheaves for maple_node_cache") and
3accabda4da1 ("mm, vma: use percpu sheaves for vm_area_struct cache"):

BUG: kernel NULL pointer dereference, address: 0000000000000040
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
PGD 0 P4D 0
Oops: Oops: 0000 [#1] SMP NOPTI
CPU: 21 UID: 0 PID: 818 Comm: kworker/u398:0 Not tainted 6.17.0-rc3.slab+ #5 PREEMPT(voluntary)
Hardware name: Dell Inc. PowerEdge R7425/02MJ3T, BIOS 1.26.0 07/30/2025
RIP: 0010:__pcs_replace_empty_main+0x44/0x1d0
Code: ec 08 48 8b 46 10 48 8b 76 08 48 85 c0 74 0b 8b 48 18 85 c9 0f 85 e5 00 00 00 65 48 63 05 e4 ee 50 02 49 8b 84 c6 e0 00 00 00 <4c> 8b 68 40 4c 89 ef e8 b0 81 ff ff 48 89 c5 48 85 c0 74 1d 48 89
RSP: 0018:ffffd2d10950bdb0 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff8a775dab74b0 RCX: 00000000ffffffff
RDX: 0000000000000cc0 RSI: ffff8a6800804000 RDI: ffff8a680004e300
RBP: ffffd2d10950be40 R08: 0000000000000060 R09: ffffffffb9367388
R10: 00000000000149e8 R11: ffff8a6f87a38000 R12: 0000000000000cc0
R13: 0000000000000cc0 R14: ffff8a680004e300 R15: 00000000000000c0
FS: 0000000000000000(0000) GS:ffff8a77a3541000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000040 CR3: 0000000e1aa24000 CR4: 00000000003506f0
Call Trace:
<TASK>
? srso_return_thunk+0x5/0x5f
? vm_area_alloc+0x1e/0x60
kmem_cache_alloc_noprof+0x4ec/0x5b0
vm_area_alloc+0x1e/0x60
create_init_stack_vma+0x26/0x210
alloc_bprm+0x139/0x200
kernel_execve+0x4a/0x140
call_usermodehelper_exec_async+0xd0/0x190
? __pfx_call_usermodehelper_exec_async+0x10/0x10
ret_from_fork+0xf0/0x110
? __pfx_call_usermodehelper_exec_async+0x10/0x10
ret_from_fork_asm+0x1a/0x30
</TASK>
Modules linked in:
CR2: 0000000000000040
---[ end trace 0000000000000000 ]---
RIP: 0010:__pcs_replace_empty_main+0x44/0x1d0
Code: ec 08 48 8b 46 10 48 8b 76 08 48 85 c0 74 0b 8b 48 18 85 c9 0f 85 e5 00 00 00 65 48 63 05 e4 ee 50 02 49 8b 84 c6 e0 00 00 00 <4c> 8b 68 40 4c 89 ef e8 b0 81 ff ff 48 89 c5 48 85 c0 74 1d 48 89
RSP: 0018:ffffd2d10950bdb0 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff8a775dab74b0 RCX: 00000000ffffffff
RDX: 0000000000000cc0 RSI: ffff8a6800804000 RDI: ffff8a680004e300
RBP: ffffd2d10950be40 R08: 0000000000000060 R09: ffffffffb9367388
R10: 00000000000149e8 R11: ffff8a6f87a38000 R12: 0000000000000cc0
R13: 0000000000000cc0 R14: ffff8a680004e300 R15: 00000000000000c0
FS: 0000000000000000(0000) GS:ffff8a77a3541000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000040 CR3: 0000000e1aa24000 CR4: 00000000003506f0
Kernel panic - not syncing: Fatal exception
Kernel Offset: 0x36a00000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
---[ end Kernel panic - not syncing: Fatal exception ]---

And noted "this is an AMD EPYC 7401 with 8 NUMA nodes configured such
that memory is only on 2 of them."

# numactl --hardware
available: 8 nodes (0-7)
node 0 cpus: 0 8 16 24 32 40 48 56 64 72 80 88
node 0 size: 0 MB
node 0 free: 0 MB
node 1 cpus: 2 10 18 26 34 42 50 58 66 74 82 90
node 1 size: 31584 MB
node 1 free: 30397 MB
node 2 cpus: 4 12 20 28 36 44 52 60 68 76 84 92
node 2 size: 0 MB
node 2 free: 0 MB
node 3 cpus: 6 14 22 30 38 46 54 62 70 78 86 94
node 3 size: 0 MB
node 3 free: 0 MB
node 4 cpus: 1 9 17 25 33 41 49 57 65 73 81 89
node 4 size: 0 MB
node 4 free: 0 MB
node 5 cpus: 3 11 19 27 35 43 51 59 67 75 83 91
node 5 size: 32214 MB
node 5 free: 31625 MB
node 6 cpus: 5 13 21 29 37 45 53 61 69 77 85 93
node 6 size: 0 MB
node 6 free: 0 MB
node 7 cpus: 7 15 23 31 39 47 55 63 71 79 87 95
node 7 size: 0 MB
node 7 free: 0 MB

Linus decoded the stacktrace to get_barn() and get_node() and determined
that kmem_cache->node[numa_mem_id()] is NULL.

The problem is due to a wrong assumption that memoryless nodes only
exist on systems with CONFIG_HAVE_MEMORYLESS_NODES, where numa_mem_id()
points to the nearest node that has memory. SLUB has been allocating its
kmem_cache_node structures only on nodes with memory and so it does with
struct node_barn.

For kmem_cache_node, get_partial_node() checks if get_node() result is
not NULL, which I assumed was for protection from a bogus node id passed
to kmalloc_node() but apparently it's also for systems where
numa_mem_id() (used when no specific node is given) might return a
memoryless node.

Fix the sheaves code the same way by checking the result of get_node()
and bailing out if it's NULL. Note that cpus on such memoryless nodes
will have degraded sheaves performance, which can be improved later,
preferably by making numa_mem_id() work properly on such systems.

Fixes: 2d517aa09bbc ("slab: add opt-in caching layer of percpu sheaves")
Reported-and-tested-by: Phil Auld <[e-mail]>
Closes: https://lore.kernel.org/all/[e-mail]/
Analyzed-by: Linus Torvalds <[e-mail]>
Link: https://lore.kernel.org/all/[e-mail]/
Signed-off-by: Vlastimil Babka <[e-mail]>

mm/slub.c

···
 	return s->node[node];
 }
 
-/* Get the barn of the current cpu's memory node */
+/*
+ * Get the barn of the current cpu's closest memory node. It may not exist on
+ * systems with memoryless nodes but without CONFIG_HAVE_MEMORYLESS_NODES
+ */
 static inline struct node_barn *get_barn(struct kmem_cache *s)
 {
-	return get_node(s, numa_mem_id())->barn;
+	struct kmem_cache_node *n = get_node(s, numa_mem_id());
+
+	if (!n)
+		return NULL;
+
+	return n->barn;
 }
 
 /*
···
 	}
 
 	barn = get_barn(s);
+	if (!barn) {
+		local_unlock(&s->cpu_sheaves->lock);
+		return NULL;
+	}
 
 	full = barn_replace_empty_sheaf(barn, pcs->main);
 
···
 	if (unlikely(pcs->main->size == 0)) {
 
 		struct slab_sheaf *full;
+		struct node_barn *barn;
 
 		if (pcs->spare && pcs->spare->size > 0) {
 			swap(pcs->main, pcs->spare);
 			goto do_alloc;
 		}
 
-		full = barn_replace_empty_sheaf(get_barn(s), pcs->main);
+		barn = get_barn(s);
+		if (!barn) {
+			local_unlock(&s->cpu_sheaves->lock);
+			return allocated;
+		}
+
+		full = barn_replace_empty_sheaf(barn, pcs->main);
 
 		if (full) {
 			stat(s, BARN_GET);
···
 {
 	struct slub_percpu_sheaves *pcs;
 	struct slab_sheaf *sheaf = NULL;
+	struct node_barn *barn;
 
 	if (unlikely(size > s->sheaf_capacity)) {
 
···
 		pcs->spare = NULL;
 		stat(s, SHEAF_PREFILL_FAST);
 	} else {
+		barn = get_barn(s);
+
 		stat(s, SHEAF_PREFILL_SLOW);
-		sheaf = barn_get_full_or_empty_sheaf(get_barn(s));
+		if (barn)
+			sheaf = barn_get_full_or_empty_sheaf(barn);
 		if (sheaf && sheaf->size)
 			stat(s, BARN_GET);
 		else
···
 	 * If the barn has too many full sheaves or we fail to refill the sheaf,
 	 * simply flush and free it.
 	 */
-	if (data_race(barn->nr_full) >= MAX_FULL_SHEAVES ||
+	if (!barn || data_race(barn->nr_full) >= MAX_FULL_SHEAVES ||
 	    refill_sheaf(s, sheaf, gfp)) {
 		sheaf_flush_unused(s, sheaf);
 		free_empty_sheaf(s, sheaf);
···
  * put the full sheaf there.
  */
 static void __pcs_install_empty_sheaf(struct kmem_cache *s,
-		struct slub_percpu_sheaves *pcs, struct slab_sheaf *empty)
+		struct slub_percpu_sheaves *pcs, struct slab_sheaf *empty,
+		struct node_barn *barn)
 {
-	struct node_barn *barn;
-
 	lockdep_assert_held(this_cpu_ptr(&s->cpu_sheaves->lock));
 
 	/* This is what we expect to find if nobody interrupted us. */
···
 		pcs->main = empty;
 		return;
 	}
-
-	barn = get_barn(s);
 
 	/*
 	 * Unlikely because if the main sheaf had space, we would have just
···
 	lockdep_assert_held(this_cpu_ptr(&s->cpu_sheaves->lock));
 
 	barn = get_barn(s);
+	if (!barn) {
+		local_unlock(&s->cpu_sheaves->lock);
+		return NULL;
+	}
+
 	put_fail = false;
 
 	if (!pcs->spare) {
···
 	}
 
 	pcs = this_cpu_ptr(s->cpu_sheaves);
-	__pcs_install_empty_sheaf(s, pcs, empty);
+	__pcs_install_empty_sheaf(s, pcs, empty, barn);
 
 	return pcs;
 }
···
 
 static void rcu_free_sheaf(struct rcu_head *head)
 {
+	struct kmem_cache_node *n;
 	struct slab_sheaf *sheaf;
-	struct node_barn *barn;
+	struct node_barn *barn = NULL;
 	struct kmem_cache *s;
 
 	sheaf = container_of(head, struct slab_sheaf, rcu_head);
···
 	 */
 	__rcu_free_sheaf_prepare(s, sheaf);
 
-	barn = get_node(s, sheaf->node)->barn;
+	n = get_node(s, sheaf->node);
+	if (!n)
+		goto flush;
+
+	barn = n->barn;
 
 	/* due to slab_free_hook() */
 	if (unlikely(sheaf->size == 0))
···
 		return;
 	}
 
+flush:
 	stat(s, BARN_PUT_FAIL);
 	sheaf_flush_unused(s, sheaf);
 
 empty:
-	if (data_race(barn->nr_empty) < MAX_EMPTY_SHEAVES) {
+	if (barn && data_race(barn->nr_empty) < MAX_EMPTY_SHEAVES) {
 		barn_put_empty_sheaf(barn, sheaf);
 		return;
 	}
···
 		}
 
 		barn = get_barn(s);
+		if (!barn) {
+			local_unlock(&s->cpu_sheaves->lock);
+			goto fail;
+		}
 
 		empty = barn_get_empty_sheaf(barn);
 
···
 		goto do_free;
 
 	barn = get_barn(s);
+	if (!barn)
+		goto no_empty;
 
 	if (!pcs->spare) {
 		empty = barn_get_empty_sheaf(barn);