slab: prevent warnings when slab obj_exts vector allocation fails

When object extension vector allocation fails, we set slab->obj_exts to
OBJEXTS_ALLOC_FAIL to indicate the failure. Later, once the vector is
successfully allocated, we will use this flag to mark codetag references
stored in that vector as empty to avoid codetag warnings.

slab_obj_exts() used to retrieve the slab->obj_exts vector pointer checks
slab->obj_exts for being either NULL or a pointer with MEMCG_DATA_OBJEXTS
bit set. However it does not handle the case when slab->obj_exts equals
OBJEXTS_ALLOC_FAIL. Add the missing condition to avoid extra warning.

Fixes: 09c4656 ("codetag: debug: introduce OBJEXTS_ALLOC_FAIL to mark failed slab_ext allocations")
Reported-by: Shakeel Butt <[email protected]>
Closes: https://lore.kernel.org/all/jftidhymri2af5u3xtcqry3cfu6aqzte3uzlznhlaylgrdztsi@5vpjnzpsemf5/
Signed-off-by: Suren Baghdasaryan <[email protected]>
Cc: [email protected] # v6.10+
Acked-by: Shakeel Butt <[email protected]>
Signed-off-by: Vlastimil Babka <[email protected]>

Author: surenbaghdasaryan

mm/slab.h

@@ -526,8 +526,12 @@ static inline struct slabobj_ext *slab_obj_exts(struct slab *slab)
 	unsigned long obj_exts = READ_ONCE(slab->obj_exts);
 
 #ifdef CONFIG_MEMCG
-	VM_BUG_ON_PAGE(obj_exts && !(obj_exts & MEMCG_DATA_OBJEXTS),
-							slab_page(slab));
+	/*
+	 * obj_exts should be either NULL, a valid pointer with
+	 * MEMCG_DATA_OBJEXTS bit set or be equal to OBJEXTS_ALLOC_FAIL.
+	 */
+	VM_BUG_ON_PAGE(obj_exts && !(obj_exts & MEMCG_DATA_OBJEXTS) &&
+		       obj_exts != OBJEXTS_ALLOC_FAIL, slab_page(slab));
 	VM_BUG_ON_PAGE(obj_exts & MEMCG_DATA_KMEM, slab_page(slab));
 #endif
 	return (struct slabobj_ext *)(obj_exts & ~OBJEXTS_FLAGS_MASK);