After Al Viro's recent swapfile cleanup, the swap_device member of

swap_info_struct became pretty much superfluous.  As we are minimizing
kdev_t usage anyway, I took the opportunity to remove swap_device
member, and replace the remaining usages with SWP_BLOCKDEV bit flag.

Adding SWP_BLOCKDEV in turn motivated a small cleanup of the
SWP_xxx bit flags and their usage.

Patch has been in light testing for a couple weeks, and
has been glanced at by Al.  "looks sane"

include/linux/swap.h

@@ -50,8 +50,12 @@ union swap_header {
 
 #include <asm/atomic.h>
 
-#define SWP_USED	1
-#define SWP_WRITEOK	3
+enum {
+	SWP_USED	= (1 << 0),	/* is slot in swap_info[] used? */
+	SWP_WRITEOK	= (1 << 1),	/* ok to write to this swap?	*/
+	SWP_BLOCKDEV	= (1 << 2),	/* is this swap a block device? */
+	SWP_ACTIVE	= (SWP_USED | SWP_WRITEOK),
+};
 
 #define SWAP_CLUSTER_MAX 32
 
@@ -63,7 +67,6 @@ union swap_header {
  */
 struct swap_info_struct {
 	unsigned int flags;
-	kdev_t swap_device;
 	spinlock_t sdev_lock;
 	struct file *swap_file;
 	unsigned short * swap_map;

mm/swapfile.c

@@ -114,7 +114,7 @@ swp_entry_t get_swap_page(void)
 
 	while (1) {
 		p = &swap_info[type];
-		if ((p->flags & SWP_WRITEOK) == SWP_WRITEOK) {
+		if ((p->flags & SWP_ACTIVE) == SWP_ACTIVE) {
 			swap_device_lock(p);
 			offset = scan_swap_map(p);
 			swap_device_unlock(p);
@@ -729,7 +729,7 @@ asmlinkage long sys_swapoff(const char * specialfile)
 	swap_list_lock();
 	for (type = swap_list.head; type >= 0; type = swap_info[type].next) {
 		p = swap_info + type;
-		if ((p->flags & SWP_WRITEOK) == SWP_WRITEOK) {
+		if ((p->flags & SWP_ACTIVE) == SWP_ACTIVE) {
 			if (p->swap_file->f_dentry == nd.dentry)
 				break;
 		}
@@ -752,7 +752,7 @@ asmlinkage long sys_swapoff(const char * specialfile)
 	}
 	nr_swap_pages -= p->pages;
 	total_swap_pages -= p->pages;
-	p->flags = SWP_USED;
+	p->flags &= ~SWP_WRITEOK;
 	swap_list_unlock();
 	unlock_kernel();
 	err = try_to_unuse(type);
@@ -770,7 +770,7 @@ asmlinkage long sys_swapoff(const char * specialfile)
 			swap_info[prev].next = p - swap_info;
 		nr_swap_pages += p->pages;
 		total_swap_pages += p->pages;
-		p->flags = SWP_WRITEOK;
+		p->flags |= SWP_WRITEOK;
 		swap_list_unlock();
 		goto out_dput;
 	}
@@ -778,7 +778,6 @@ asmlinkage long sys_swapoff(const char * specialfile)
 	swap_device_lock(p);
 	swap_file = p->swap_file;
 	p->swap_file = NULL;
-	p->swap_device = NODEV;
 	p->max = 0;
 	swap_map = p->swap_map;
 	p->swap_map = NULL;
@@ -822,7 +821,8 @@ int get_swaparea_info(char *buf)
 				}
 			len += sprintf(buf + len, "%-39s %s\t%d\t%d\t%d\n",
 				       path,
-				       !kdev_none(ptr->swap_device) ? "partition" : "file\t",
+				       (ptr->flags & SWP_BLOCKDEV) ?
+				       		"partition" : "file\t",
 				       ptr->pages << (PAGE_SHIFT - 10),
 				       usedswap << (PAGE_SHIFT - 10),
 				       ptr->prio);
@@ -837,8 +837,9 @@ int is_swap_partition(kdev_t dev) {
 	int i;
 
 	for (i = 0 ; i < nr_swapfiles ; i++, ptr++) {
-		if (ptr->flags & SWP_USED)
-			if (kdev_same(ptr->swap_device, dev))
+		if ((ptr->flags & SWP_USED) &&
+		    (ptr->flags & SWP_BLOCKDEV) &&
+		    (kdev_same(ptr->swap_file->f_dentry->d_inode->i_rdev, dev)))
 			return 1;
 	}
 	return 0;
@@ -883,7 +884,6 @@ asmlinkage long sys_swapon(const char * specialfile, int swap_flags)
 		nr_swapfiles = type+1;
 	p->flags = SWP_USED;
 	p->swap_file = NULL;
-	p->swap_device = NODEV;
 	p->swap_map = NULL;
 	p->lowest_bit = 0;
 	p->highest_bit = 0;
@@ -911,8 +911,10 @@ asmlinkage long sys_swapon(const char * specialfile, int swap_flags)
 
 	error = -EINVAL;
 	if (S_ISBLK(swap_file->f_dentry->d_inode->i_mode)) {
-		p->swap_device = swap_file->f_dentry->d_inode->i_rdev;
-		set_blocksize(p->swap_device, PAGE_SIZE);
+		error = set_blocksize(swap_file->f_dentry->d_inode->i_rdev,
+				      PAGE_SIZE);
+		if (error < 0)
+			goto bad_swap;
 	} else if (!S_ISREG(swap_file->f_dentry->d_inode->i_mode))
 		goto bad_swap;
 
@@ -1035,7 +1037,9 @@ asmlinkage long sys_swapon(const char * specialfile, int swap_flags)
 	swap_list_lock();
 	swap_device_lock(p);
 	p->max = maxpages;
-	p->flags = SWP_WRITEOK;
+	p->flags = SWP_ACTIVE;
+	if (S_ISBLK(swap_file->f_dentry->d_inode->i_mode))
+		p->flags |= SWP_BLOCKDEV;
 	p->pages = nr_good_pages;
 	nr_swap_pages += nr_good_pages;
 	total_swap_pages += nr_good_pages;
@@ -1064,7 +1068,6 @@ asmlinkage long sys_swapon(const char * specialfile, int swap_flags)
 bad_swap_2:
 	swap_list_lock();
 	swap_map = p->swap_map;
-	p->swap_device = NODEV;
 	p->swap_file = NULL;
 	p->swap_map = NULL;
 	p->flags = 0;
@@ -1090,7 +1093,7 @@ void si_swapinfo(struct sysinfo *val)
 	swap_list_lock();
 	for (i = 0; i < nr_swapfiles; i++) {
 		unsigned int j;
-		if (swap_info[i].flags != SWP_USED)
+		if (!(swap_info[i].flags & SWP_USED))
 			continue;
 		for (j = 0; j < swap_info[i].max; ++j) {
 			switch (swap_info[i].swap_map[j]) {