kasan: clean up __kasan_mempool_poison_object

Reorganize the code and reword the comment in
__kasan_mempool_poison_object to improve the code readability.

Link: https://lkml.kernel.org/r/4f6fc8840512286c1a96e16e86901082c671677d.1703024586.git.andreyknvl@google.comSigned-off-by: Andrey Konovalov <andreyknvl@google.com>
Cc: Alexander Lobakin <alobakin@pm.me>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Breno Leitao <leitao@debian.org>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Evgenii Stepanov <eugenis@google.com>
Cc: Marco Elver <elver@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/kasan/common.c

@@ -457,27 +457,22 @@ void __kasan_mempool_unpoison_pages(struct page *page, unsigned int order,
 
 bool __kasan_mempool_poison_object(void *ptr, unsigned long ip)
 {
-	struct folio *folio;
-
-	folio = virt_to_folio(ptr);
+	struct folio *folio = virt_to_folio(ptr);
+	struct slab *slab;
 
 	/*
-	 * Even though this function is only called for kmem_cache_alloc and
-	 * kmalloc backed mempool allocations, those allocations can still be
-	 * !PageSlab() when the size provided to kmalloc is larger than
-	 * KMALLOC_MAX_SIZE, and kmalloc falls back onto page_alloc.
+	 * This function can be called for large kmalloc allocation that get
+	 * their memory from page_alloc. Thus, the folio might not be a slab.
 	 */
 	if (unlikely(!folio_test_slab(folio))) {
 		if (check_page_allocation(ptr, ip))
 			return false;
 		kasan_poison(ptr, folio_size(folio), KASAN_PAGE_FREE, false);
 		return true;
-	} else {
-		struct slab *slab = folio_slab(folio);
-
-		return !____kasan_slab_free(slab->slab_cache, ptr, ip,
-						false, false);
 	}
+
+	slab = folio_slab(folio);
+	return !____kasan_slab_free(slab->slab_cache, ptr, ip, false, false);
 }
 
 void __kasan_mempool_unpoison_object(void *ptr, size_t size, unsigned long ip)