[PATCH] make loop set right queue restrictions

This makes loop honor the queue restrictions by basically stacking all
of those, and mirroring the merge_bvec_fn() on the target queue. It also
switches loop to use per-loop device queues, since that is the only sane
way to do this from a performance POV. Also, in principle I find it to
be much nicer if every distinct block device has its own queue.

drivers/block/loop.c

@@ -500,15 +500,13 @@ bio_transfer(struct loop_device *lo, struct bio *to_bio,
 static int loop_make_request(request_queue_t *q, struct bio *old_bio)
 {
 	struct bio *new_bio = NULL;
-	struct loop_device *lo;
+	struct loop_device *lo = q->queuedata;
 	unsigned long IV;
 	int rw = bio_rw(old_bio);
-	int unit = minor(to_kdev_t(old_bio->bi_bdev->bd_dev));
 
-	if (unit >= max_loop)
+	if (!lo)
 		goto out;
 
-	lo = &loop_dev[unit];
 	spin_lock_irq(&lo->lo_lock);
 	if (lo->lo_state != Lo_bound)
 		goto inactive;
@@ -712,6 +710,29 @@ static int loop_set_fd(struct loop_device *lo, struct file *lo_file,
 	set_blocksize(bdev, block_size(lo_device));
 
 	lo->lo_bio = lo->lo_biotail = NULL;
+
+	/*
+	 * set queue make_request_fn, and add limits based on lower level
+	 * device
+	 */
+	blk_queue_make_request(&lo->lo_queue, loop_make_request);
+	blk_queue_bounce_limit(&lo->lo_queue, BLK_BOUNCE_HIGH);
+	lo->lo_queue.queuedata = lo;
+
+	/*
+	 * we remap to a block device, make sure we correctly stack limits
+	 */
+	if (S_ISBLK(inode->i_mode)) {
+		request_queue_t *q = bdev_get_queue(lo_device);
+
+		blk_queue_max_sectors(&lo->lo_queue, q->max_sectors);
+		blk_queue_max_phys_segments(&lo->lo_queue,q->max_phys_segments);
+		blk_queue_max_hw_segments(&lo->lo_queue, q->max_hw_segments);
+		blk_queue_max_segment_size(&lo->lo_queue, q->max_segment_size);
+		blk_queue_segment_boundary(&lo->lo_queue, q->seg_boundary_mask);
+		blk_queue_merge_bvec(&lo->lo_queue, q->merge_bvec_fn);
+	}
+
 	kernel_thread(loop_thread, lo, CLONE_FS | CLONE_FILES | CLONE_SIGHAND);
 	down(&lo->lo_sem);
 
@@ -991,6 +1012,16 @@ int loop_unregister_transfer(int number)
 EXPORT_SYMBOL(loop_register_transfer);
 EXPORT_SYMBOL(loop_unregister_transfer);
 
+request_queue_t *loop_get_queue(kdev_t dev)
+{
+	int minor = minor(dev);
+
+	if (minor < max_loop)
+		return &loop_dev[minor].lo_queue;
+
+	return NULL;
+}
+
 int __init loop_init(void) 
 {
 	int	i;
@@ -1023,9 +1054,6 @@ int __init loop_init(void)
 
 	names = kmalloc(max_loop * 8, GFP_KERNEL);
 
-	blk_queue_make_request(BLK_DEFAULT_QUEUE(MAJOR_NR), loop_make_request);
-	blk_queue_bounce_limit(BLK_DEFAULT_QUEUE(MAJOR_NR), BLK_BOUNCE_HIGH);
-
 	for (i = 0; i < max_loop; i++) {
 		struct loop_device *lo = &loop_dev[i];
 		struct gendisk *disk = disks + i;
@@ -1044,6 +1072,8 @@ int __init loop_init(void)
 		add_disk(disk);
 	}
 
+	blk_dev[LOOP_MAJOR].queue = loop_get_queue;
+
 	printk(KERN_INFO "loop: loaded (max %d devices)\n", max_loop);
 	return 0;
 

include/linux/loop.h

@@ -56,6 +56,8 @@ struct loop_device {
 	struct semaphore	lo_ctl_mutex;
 	struct semaphore	lo_bh_mutex;
 	atomic_t		lo_pending;
+
+	request_queue_t		lo_queue;
 };
 
 typedef	int (* transfer_proc_t)(struct loop_device *, int cmd,