userswap: add page_count() check for swap-out VA

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6CAIM

--------------------------------

Add page_count() check for swap-out VA to make sure that no other kernel
mechanism is using this physical page.
Add lru_add_drain_all() before swap-out VA to correct page_count() of
swap-out pages.
Signed-off-by: NZhangPeng <zhangpeng362@huawei.com>

mm/userswap.c

@@ -139,7 +139,8 @@ static unsigned long pages_can_be_swapped(struct mm_struct *mm,
 		 * Check that no O_DIRECT or similar I/O is in progress on the
 		 * page
 		 */
-		if (page_mapcount(page) > 1) {
+		if (page_mapcount(page) > 1 ||
+		    page_mapcount(page) + 1 != page_count(page)) {
 			ret = -EBUSY;
 			goto out_err;
 		}
@@ -378,6 +379,7 @@ unsigned long uswap_mremap(unsigned long old_addr, unsigned long old_len,
 	if (old_addr + old_len > new_addr && new_addr + new_len > old_addr)
 		return ret;
 
+	lru_add_drain_all();
 	down_read(&mm->mmap_lock);
 	ret = pages_can_be_swapped(mm, old_addr, len, &pages);
 	if (ret) {