emm: mm/swap: misc: print a warning for swapping on block file over fs

Upstream: pending

Looking at setup_swap_extents, swap_activate never get called if the
overlying file for swap is a block file.

And swap_activate is the only place that sets SWP_FS_OPS, so a block
file over filesystem will make SWP_FS_OPS never take effect and swap_rw
get ignored.

Although there is currently no such mis-combination in kernel, make this
a warn so things are clear.

Signed-off-by: Kairui Song <kasong@tencent.com>

mm/swapfile.c

@@ -2802,6 +2802,11 @@ static int claim_swapfile(struct swap_info_struct *p, struct inode *inode)
 	int error;
 
 	if (S_ISBLK(inode->i_mode)) {
+#ifdef CONFIG_ENHANCED_MM
+		WARN(p->swap_file->f_mapping->a_ops->swap_activate,
+				"Swapping on block file over filesystem %s, file system operations may get bypassed unexpectedly and lead to data loss.\n",
+				p->swap_file->f_inode->i_sb->s_id);
+#endif
 		p->bdev = blkdev_get_by_dev(inode->i_rdev,
 				BLK_OPEN_READ | BLK_OPEN_WRITE, p, NULL);
 		if (IS_ERR(p->bdev)) {