mm/slob: Remove unnecessary page_mapcount_reset() function call

After commit 401fb12c68 ("mm/sl*b: Differentiate struct slab fields by
sl*b implementations"), we can reorder fields of struct slab depending
on slab allocator.

For now, page_mapcount_reset() is called because page->_mapcount and
slab->units have same offset. But this is not necessary for struct slab.
Use unused field for units instead.

Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Link: https://lore.kernel.org/r/20211212065241.GA886691@odroid

mm/slab.h

@@ -50,8 +50,8 @@ struct slab {
 	struct list_head slab_list;
 	void *__unused_1;
 	void *freelist;		/* first free block */
-	void *__unused_2;
-	int units;
+	long units;
+	unsigned int __unused_2;
 
 #else
 #error "Unexpected slab allocator configured"

mm/slob.c

@@ -404,7 +404,6 @@ static void slob_free(void *block, int size)
 			clear_slob_page_free(sp);
 		spin_unlock_irqrestore(&slob_lock, flags);
 		__folio_clear_slab(slab_folio(sp));
-		page_mapcount_reset(slab_page(sp));
 		slob_free_pages(b, 0);
 		return;
 	}