[PATCH] dm-multipath: fix stall on noflush suspend/resume

Allow noflush suspend/resume of device-mapper device only for the case
where the device size is unchanged.

Otherwise, dm-multipath devices can stall when resumed if noflush was used
when suspending them, all paths have failed and queue_if_no_path is set.

Explanation:
1. Something is doing fsync() on the block dev,
holding inode->i_sem
2. The fsync write is blocked by all-paths-down and queue_if_no_path
3. Someone requests to suspend the dm device with noflush.
Pending writes are left in queue.
4. In the middle of dm_resume(), __bind() tries to get
inode->i_sem to do __set_size() and waits forever.

'noflush suspend' is a new device-mapper feature introduced in
early 2.6.20. So I hope the fix being included before 2.6.20 is
released.

Example of reproducer:
1. Create a multipath device by dmsetup
2. Fail all paths during mkfs
3. Do dmsetup suspend --noflush and load new map with healthy paths
4. Do dmsetup resume

Signed-off-by: Jun'ichi Nomura <[e-mail]>
Acked-by: Alasdair G Kergon <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/md/dm.c

···
 	if (size != get_capacity(md->disk))
 		memset(&md->geometry, 0, sizeof(md->geometry));
 
-	__set_size(md, size);
+	if (md->suspended_bdev)
+		__set_size(md, size);
 	if (size == 0)
 		return 0;
 
···
 	if (!dm_suspended(md))
 		goto out;
 
+	/* without bdev, the device size cannot be changed */
+	if (!md->suspended_bdev)
+		if (get_capacity(md->disk) != dm_table_get_size(table))
+			goto out;
+
 	__unbind(md);
 	r = __bind(md, table);
 
···
 	/* This does not get reverted if there's an error later. */
 	dm_table_presuspend_targets(map);
 
-	md->suspended_bdev = bdget_disk(md->disk, 0);
-	if (!md->suspended_bdev) {
-		DMWARN("bdget failed in dm_suspend");
-		r = -ENOMEM;
-		goto flush_and_out;
+	/* bdget() can stall if the pending I/Os are not flushed */
+	if (!noflush) {
+		md->suspended_bdev = bdget_disk(md->disk, 0);
+		if (!md->suspended_bdev) {
+			DMWARN("bdget failed in dm_suspend");
+			r = -ENOMEM;
+			goto flush_and_out;
+		}
 	}
 
 	/*
···
 
 	unlock_fs(md);
 
-	bdput(md->suspended_bdev);
-	md->suspended_bdev = NULL;
+	if (md->suspended_bdev) {
+		bdput(md->suspended_bdev);
+		md->suspended_bdev = NULL;
+	}
 
 	clear_bit(DMF_SUSPENDED, &md->flags);