mm, swap: unlock inode in error path of claim_swapfile

claim_swapfile() currently keeps the inode locked when it is successful, or
the file is already swapfile (with -EBUSY). And, on the other error cases,
it does not lock the inode.

This inconsistency of the lock state and return value is quite confusing
and actually causing a bad unlock balance as below in the "bad_swap"
section of __do_sys_swapon().

This commit fixes this issue by unlocking the inode on the error path. It
also reverts blocksize and releases bdev, so that the caller can safely
forget about the inode.

    =====================================
    WARNING: bad unlock balance detected!
    5.5.0-rc7+ torvalds#176 Not tainted
    -------------------------------------
    swapon/4294 is trying to release lock (&sb->s_type->i_mutex_key) at:
    [<ffffffff8173a6eb>] __do_sys_swapon+0x94b/0x3550
    but there are no more locks to release!

    other info that might help us debug this:
    no locks held by swapon/4294.

    stack backtrace:
    CPU: 5 PID: 4294 Comm: swapon Not tainted 5.5.0-rc7-BTRFS-ZNS+ torvalds#176
    Hardware name: ASUS All Series/H87-PRO, BIOS 2102 07/29/2014
    Call Trace:
     dump_stack+0xa1/0xea
     ? __do_sys_swapon+0x94b/0x3550
     print_unlock_imbalance_bug.cold+0x114/0x123
     ? __do_sys_swapon+0x94b/0x3550
     lock_release+0x562/0xed0
     ? kvfree+0x31/0x40
     ? lock_downgrade+0x770/0x770
     ? kvfree+0x31/0x40
     ? rcu_read_lock_sched_held+0xa1/0xd0
     ? rcu_read_lock_bh_held+0xb0/0xb0
     up_write+0x2d/0x490
     ? kfree+0x293/0x2f0
     __do_sys_swapon+0x94b/0x3550
     ? putname+0xb0/0xf0
     ? kmem_cache_free+0x2e7/0x370
     ? do_sys_open+0x184/0x3e0
     ? generic_max_swapfile_size+0x40/0x40
     ? do_syscall_64+0x27/0x4b0
     ? entry_SYSCALL_64_after_hwframe+0x49/0xbe
     ? lockdep_hardirqs_on+0x38c/0x590
     __x64_sys_swapon+0x54/0x80
     do_syscall_64+0xa4/0x4b0
     entry_SYSCALL_64_after_hwframe+0x49/0xbe
    RIP: 0033:0x7f15da0a0dc7

Fixes: 1638045 ("mm: set S_SWAPFILE on blockdev swap devices")
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>

mm/swapfile.c

@@ -2886,24 +2886,37 @@ static int claim_swapfile(struct swap_info_struct *p, struct inode *inode)
 		p->old_block_size = block_size(p->bdev);
 		error = set_blocksize(p->bdev, PAGE_SIZE);
 		if (error < 0)
-			return error;
+			goto err;
 		/*
 		 * Zoned block devices contain zones that have a sequential
 		 * write only restriction.  Hence zoned block devices are not
 		 * suitable for swapping.  Disallow them here.
 		 */
-		if (blk_queue_is_zoned(p->bdev->bd_queue))
-			return -EINVAL;
+		if (blk_queue_is_zoned(p->bdev->bd_queue)) {
+			error = -EINVAL;
+			goto err;
+		}
 		p->flags |= SWP_BLKDEV;
 	} else if (S_ISREG(inode->i_mode)) {
 		p->bdev = inode->i_sb->s_bdev;
 	}
 
 	inode_lock(inode);
-	if (IS_SWAPFILE(inode))
-		return -EBUSY;
+	if (IS_SWAPFILE(inode)) {
+		inode_unlock(inode);
+		error = -EBUSY;
+		goto err;
+	}
 
 	return 0;
+
+err:
+	if (S_ISBLK(inode->i_mode)) {
+		set_blocksize(p->bdev, p->old_block_size);
+		blkdev_put(p->bdev, FMODE_READ | FMODE_WRITE | FMODE_EXCL);
+	}
+
+	return error;
 }
 
 
@@ -3157,10 +3170,12 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	mapping = swap_file->f_mapping;
 	inode = mapping->host;
 
-	/* If S_ISREG(inode->i_mode) will do inode_lock(inode); */
+	/* do inode_lock(inode); */
 	error = claim_swapfile(p, inode);
-	if (unlikely(error))
+	if (unlikely(error)) {
+		inode = NULL;
 		goto bad_swap;
+	}
 
 	/*
 	 * Read the swap header.