[PATCH] slab: prevent infinite loop in kmalloc_nolock() with debugging

[Vlastimil Babka]

In review of a followup work, Harry noticed a potential infinite loop.
Upon closed inspection, it already exists for kmalloc_nolock() on a
cache with debugging enabled, since commit af92793e52c3 ("slab:
Introduce kmalloc_nolock() and kfree_nolock().")

When alloc_single_from_new_slab() fails to trylock node list_lock, we
keep retrying to get partial slab or allocate a new slab. If we indeed
interrupted somebody holding the list_lock, the trylock fill fail
deterministically and we end up allocating and defer-freeing slabs
indefinitely with no progress.

To fix it, fail the allocation if spinning is not allowed. This is
acceptable in the restricted context of kmalloc_nolock(), especially
with debugging enabled.

Reported-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
Closes: https://lore.kernel.org/all/aQLqZjjq1SPD3Fml@hyeyoo/
Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
as we discussed in the linked thread, 6.18 hotfix to be included in
slab/for-next-fixes
---
 mm/slub.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/slub.c b/mm/slub.c
index d4367f25b20d..f1a5373eee7b 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -4666,8 +4666,12 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
 	if (kmem_cache_debug(s)) {
 		freelist = alloc_single_from_new_slab(s, slab, orig_size, gfpflags);
 
-		if (unlikely(!freelist))
+		if (unlikely(!freelist)) {
+			/* This could cause an endless loop. Fail instead. */
+			if (!allow_spin)
+				return NULL;
 			goto new_objects;
+		}
 
 		if (s->flags & SLAB_STORE_USER)
 			set_track(s, freelist, TRACK_ALLOC, addr,

---
base-commit: 6146a0f1dfae5d37442a9ddcba012add260bceb0
change-id: 20251103-fix-nolock-loop-854e0101672f

Best regards,
-- 
Vlastimil Babka <vbabka@xxxxxxx>