swap: Add swap slot free callback to block_device_operations

This callback is required when RAM based devices are used as swap disks.
One such device is ramzswap which is used as compressed in-memory swap
disk.  For such devices, we need a callback as soon as a swap slot is no
longer used to allow freeing memory allocated for this slot.  Without this
callback, stale data can quickly accumulate in memory defeating the whole
purpose of such devices.
Signed-off-by: Nitin Gupta <ngupta@vflare.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Nigel Cunningham <nigel@tuxonice.net>
Acked-by: Pekka Enberg <penberg@cs.helsinki.fi>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

include/linux/blkdev.h

@@ -1287,6 +1287,8 @@ struct block_device_operations {
 						unsigned long long);
 	int (*revalidate_disk) (struct gendisk *);
 	int (*getgeo)(struct block_device *, struct hd_geometry *);
+	/* this callback is with swap_lock and sometimes page table lock held */
+	void (*swap_slot_free_notify) (struct block_device *, unsigned long);
 	struct module *owner;
 };
 

mm/swapfile.c

@@ -574,6 +574,7 @@ static unsigned char swap_entry_free(struct swap_info_struct *p,
 
 	/* free if no reference */
 	if (!usage) {
+		struct gendisk *disk = p->bdev->bd_disk;
 		if (offset < p->lowest_bit)
 			p->lowest_bit = offset;
 		if (offset > p->highest_bit)
@@ -583,6 +584,9 @@ static unsigned char swap_entry_free(struct swap_info_struct *p,
 			swap_list.next = p->type;
 		nr_swap_pages++;
 		p->inuse_pages--;
+		if ((p->flags & SWP_BLKDEV) &&
+				disk->fops->swap_slot_free_notify)
+			disk->fops->swap_slot_free_notify(p->bdev, offset);
 	}
 
 	return usage;