[PATCH RFC 6/9] mm/slub: materialize trie-backed stack depot traces

[Caleb Kan <calebkan1106@gmail.com>]

From: Caleb Kan <ckan@cloudflare.com>

SLUB owner tracking stores allocation and free stacks as persistent stack
depot handles. Trie-backed handles do not expose contiguous stack-record
entries, so __kmem_obj_info() and the alloc_traces and free_traces debugfs
files cannot use stack_depot_fetch().

Use stack_depot_fetch_into() with TRACK_ADDRS_COUNT-sized local arrays.
This matches the save-side limit. Keep the existing KS_ADDRS_COUNT copy
limit and debugfs formatting unchanged for hash-backed handles. Continue
to copy or print no frames when the fetch returns zero.

Signed-off-by: Caleb Kan <ckan@cloudflare.com>
---
 mm/slub.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 422bc3e12c02..138c3bc473c9 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -8093,12 +8093,12 @@ void __kmem_obj_info(struct kmem_obj_info *kpp, void *object, struct slab *slab)
 #ifdef CONFIG_STACKDEPOT
 	{
 		depot_stack_handle_t handle;
-		unsigned long *entries;
+		unsigned long entries[TRACK_ADDRS_COUNT];
 		unsigned int nr_entries;
 
 		handle = READ_ONCE(trackp->handle);
 		if (handle) {
-			nr_entries = stack_depot_fetch(handle, &entries);
+			nr_entries = stack_depot_fetch_into(handle, entries, ARRAY_SIZE(entries));
 			for (i = 0; i < KS_ADDRS_COUNT && i < nr_entries; i++)
 				kpp->kp_stack[i] = (void *)entries[i];
 		}
@@ -8106,7 +8106,7 @@ void __kmem_obj_info(struct kmem_obj_info *kpp, void *object, struct slab *slab)
 		trackp = get_track(s, objp, TRACK_FREE);
 		handle = READ_ONCE(trackp->handle);
 		if (handle) {
-			nr_entries = stack_depot_fetch(handle, &entries);
+			nr_entries = stack_depot_fetch_into(handle, entries, ARRAY_SIZE(entries));
 			for (i = 0; i < KS_ADDRS_COUNT && i < nr_entries; i++)
 				kpp->kp_free_stack[i] = (void *)entries[i];
 		}
@@ -9815,12 +9815,14 @@ static int slab_debugfs_show(struct seq_file *seq, void *v)
 #ifdef CONFIG_STACKDEPOT
 		{
 			depot_stack_handle_t handle;
-			unsigned long *entries;
+			unsigned long entries[TRACK_ADDRS_COUNT];
 			unsigned int nr_entries, j;
 
 			handle = READ_ONCE(l->handle);
 			if (handle) {
-				nr_entries = stack_depot_fetch(handle, &entries);
+				nr_entries =
+					stack_depot_fetch_into(handle, entries,
+							       ARRAY_SIZE(entries));
 				seq_puts(seq, "\n");
 				for (j = 0; j < nr_entries; j++)
 					seq_printf(seq, "        %pS\n", (void *)entries[j]);

-- 
Git-155)