slab: wrap debug slab validation in validate_slab_ptr()

This will make it clear where we currently cast struct slab to folio
only to check the slab type, and allow to change the implementation
later with memdesc conversion.

For now use a struct page based implementation instead of struct folio
to be compatible with further upcoming changes.

Reviewed-by: Harry Yoo <[email protected]>
Signed-off-by: Vlastimil Babka <[email protected]>

Author: tehcaster

mm/slub.c

@@ -821,6 +821,15 @@ static inline unsigned int get_orig_size(struct kmem_cache *s, void *object)
 	return *(unsigned int *)p;
 }
 
+/*
+ * For debugging context when we want to check if the struct slab pointer
+ * appears to be valid.
+ */
+static inline bool validate_slab_ptr(struct slab *slab)
+{
+	return PageSlab(slab_page(slab));
+}
+
 #ifdef CONFIG_SLUB_DEBUG
 static unsigned long object_map[BITS_TO_LONGS(MAX_OBJS_PER_PAGE)];
 static DEFINE_SPINLOCK(object_map_lock);
@@ -1453,7 +1462,7 @@ static int check_slab(struct kmem_cache *s, struct slab *slab)
 {
 	int maxobj;
 
-	if (!folio_test_slab(slab_folio(slab))) {
+	if (!validate_slab_ptr(slab)) {
 		slab_err(s, slab, "Not a valid slab page");
 		return 0;
 	}
@@ -1653,7 +1662,7 @@ static noinline bool alloc_debug_processing(struct kmem_cache *s,
 	return true;
 
 bad:
-	if (folio_test_slab(slab_folio(slab))) {
+	if (validate_slab_ptr(slab)) {
 		/*
 		 * If this is a slab page then lets do the best we can
 		 * to avoid issues in the future. Marking all objects
@@ -2818,7 +2827,7 @@ static void *alloc_single_from_partial(struct kmem_cache *s,
 	slab->inuse++;
 
 	if (!alloc_debug_processing(s, slab, object, orig_size)) {
-		if (folio_test_slab(slab_folio(slab)))
+		if (validate_slab_ptr(slab))
 			remove_partial(n, slab);
 		return NULL;
 	}