Linux 4.14 compat: blk_queue_stackable()

The blk_queue_stackable() function was replaced in the 4.14 kernel
by queue_is_rq_based(), commit torvalds/linux@5fdee212.  This change
resulted in the default elevator being used which can negatively
impact performance.

Rather than adding additional compatibility code to detect the
new interface unconditionally attempt to set the elevator.  Since
we expect this to fail for block devices without an elevator the
error message has been moved in to zfs_dbgmsg().

Finally, it was observed that the elevator_change() was removed
from the 4.12 kernel, commit torvalds/linux@c033269.  Update the
comment to clearly specify which are expected to export the
elevator_change() symbol.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7645

config/kernel-elevator-change.m4

@@ -1,6 +1,6 @@
 dnl #
-dnl # 2.6.36 API change
+dnl # 2.6.36 API, exported elevator_change() symbol
-dnl # Verify the elevator_change() symbol is available.
+dnl # 4.12 API, removed elevator_change() symbol
 dnl #
 AC_DEFUN([ZFS_AC_KERNEL_ELEVATOR_CHANGE], [
 	AC_MSG_CHECKING([whether elevator_change() is available])

include/linux/blkdev_compat.h

@@ -105,17 +105,6 @@ blk_queue_set_write_cache(struct request_queue *q, bool wc, bool fua)
 #define	blk_fs_request(rq)	((rq)->cmd_type == REQ_TYPE_FS)
 #endif
 
-/*
- * 2.6.27 API change,
- * The blk_queue_stackable() queue flag was added in 2.6.27 to handle dm
- * stacking drivers.  Prior to this request stacking drivers were detected
- * by checking (q->request_fn == NULL), for earlier kernels we revert to
- * this legacy behavior.
- */
-#ifndef blk_queue_stackable
-#define	blk_queue_stackable(q)	((q)->request_fn == NULL)
-#endif
-
 /*
  * 2.6.34 API change,
  * The blk_queue_max_hw_sectors() function replaces blk_queue_max_sectors().

module/zfs/vdev_disk.c

@@ -168,23 +168,20 @@ vdev_elevator_switch(vdev_t *v, char *elevator)
 	if (!v->vdev_wholedisk && strncmp(device, "dm-", 3) != 0)
 		return;
 
-	/* Skip devices without schedulers (loop, ram, dm, etc) */
-	if (!q->elevator || !blk_queue_stackable(q))
-		return;
-
 	/* Leave existing scheduler when set to "none" */
 	if ((strncmp(elevator, "none", 4) == 0) && (strlen(elevator) == 4))
 		return;
 
+	/*
+	 * The elevator_change() function was available in kernels from
+	 * 2.6.36 to 4.11.  When not available fall back to using the user
+	 * mode helper functionality to set the elevator via sysfs.  This
+	 * requires /bin/echo and sysfs to be mounted which may not be true
+	 * early in the boot process.
+	 */
 #ifdef HAVE_ELEVATOR_CHANGE
 	error = elevator_change(q, elevator);
 #else
-	/*
-	 * For pre-2.6.36 kernels elevator_change() is not available.
-	 * Therefore we fall back to using a usermodehelper to echo the
-	 * elevator into sysfs;  This requires /bin/echo and sysfs to be
-	 * mounted which may not be true early in the boot process.
-	 */
 #define	SET_SCHEDULER_CMD \
 	"exec 0</dev/null " \
 	"     1>/sys/block/%s/queue/scheduler " \
@@ -198,9 +195,10 @@ vdev_elevator_switch(vdev_t *v, char *elevator)
 	error = call_usermodehelper(argv[0], argv, envp, UMH_WAIT_PROC);
 	strfree(argv[2]);
 #endif /* HAVE_ELEVATOR_CHANGE */
-	if (error)
+	if (error) {
-		printk("ZFS: Unable to set \"%s\" scheduler for %s (%s): %d\n",
+		zfs_dbgmsg("Unable to set \"%s\" scheduler for %s (%s): %d\n",
 		    elevator, v->vdev_path, device, error);
+	}
 }
 
 /*