ksm: use GET_KSM_PAGE_NOLOCK to get ksm page in remove_rmap_item_from_tree()

It's unnecessary to lock the page when get ksm page if we're going to
remove the rmap item as page migration is irrelevant in this case.  Use
GET_KSM_PAGE_NOLOCK instead to save some page lock cycles.

Link: https://lkml.kernel.org/r/20210330140228.45635-3-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/ksm.c

@@ -778,12 +778,11 @@ static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
 		struct page *page;
 
 		stable_node = rmap_item->head;
-		page = get_ksm_page(stable_node, GET_KSM_PAGE_LOCK);
+		page = get_ksm_page(stable_node, GET_KSM_PAGE_NOLOCK);
 		if (!page)
 			goto out;
 
 		hlist_del(&rmap_item->hlist);
-		unlock_page(page);
 		put_page(page);
 
 		if (!hlist_empty(&stable_node->hlist))