slab: prevent infinite loop in kmalloc_nolock() with debugging

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@oracle.com>
Closes: https://lore.kernel.org/all/aQLqZjjq1SPD3Fml@hyeyoo/
Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
Acked-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Link: https://patch.msgid.link/20251103-fix-nolock-loop-v1-1-6e2b3e82b9da@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

diff --git a/mm/slub.c b/mm/slub.c
index d4367f25b20db9f9917e743334b06b8965f9fab6..f1a5373eee7b7d3fe76b457e2d21e4d66d8873b4 100644 (file)
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -4666,8 +4666,12 @@ new_objects:
        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,