swapon(2): open swap with O_EXCL

... eliminating the need to reopen block devices so they could be
exclusively held.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Author: Al Viro

include/linux/swap.h

@@ -298,7 +298,6 @@ struct swap_info_struct {
 	unsigned int __percpu *cluster_next_cpu; /*percpu index for next allocation */
 	struct percpu_cluster __percpu *percpu_cluster; /* per cpu's swap location */
 	struct rb_root swap_extent_root;/* root of the swap extent rbtree */
-	struct file *bdev_file;		/* open handle of the bdev */
 	struct block_device *bdev;	/* swap device or bdev of swap file */
 	struct file *swap_file;		/* seldom referenced */
 	struct completion comp;		/* seldom referenced */

mm/swapfile.c

@@ -2550,10 +2550,6 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 	exit_swap_address_space(p->type);
 
 	inode = mapping->host;
-	if (p->bdev_file) {
-		fput(p->bdev_file);
-		p->bdev_file = NULL;
-	}
 
 	inode_lock(inode);
 	inode->i_flags &= ~S_SWAPFILE;
@@ -2780,14 +2776,7 @@ static struct swap_info_struct *alloc_swap_info(void)
 static int claim_swapfile(struct swap_info_struct *p, struct inode *inode)
 {
 	if (S_ISBLK(inode->i_mode)) {
-		p->bdev_file = bdev_file_open_by_dev(inode->i_rdev,
-				BLK_OPEN_READ | BLK_OPEN_WRITE, p, NULL);
-		if (IS_ERR(p->bdev_file)) {
-			int error = PTR_ERR(p->bdev_file);
-			p->bdev_file = NULL;
-			return error;
-		}
-		p->bdev = file_bdev(p->bdev_file);
+		p->bdev = I_BDEV(inode);
 		/*
 		 * Zoned block devices contain zones that have a sequential
 		 * write only restriction.  Hence zoned block devices are not
@@ -3028,7 +3017,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 		name = NULL;
 		goto bad_swap;
 	}
-	swap_file = file_open_name(name, O_RDWR|O_LARGEFILE, 0);
+	swap_file = file_open_name(name, O_RDWR | O_LARGEFILE | O_EXCL, 0);
 	if (IS_ERR(swap_file)) {
 		error = PTR_ERR(swap_file);
 		swap_file = NULL;
@@ -3225,10 +3214,6 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	p->percpu_cluster = NULL;
 	free_percpu(p->cluster_next_cpu);
 	p->cluster_next_cpu = NULL;
-	if (p->bdev_file) {
-		fput(p->bdev_file);
-		p->bdev_file = NULL;
-	}
 	inode = NULL;
 	destroy_swap_extents(p);
 	swap_cgroup_swapoff(p->type);