[dm-devel] [PATCH] dm mpath: only retry ioctl if queue_if_no_path was configured

[Mike Snitzer]

On Sat, Sep 08 2012 at 12:50pm -0400,
Mikulas Patocka <mpatocka at redhat.com> wrote:
 
> It's hard to say what should be done correctly ... if you create a 
> multipath device with "queue_if_no_path" and no active path, it should 
> delay all requests until a path becomes available ... and it is doing 
> that.
> 
> Maybe we could move the waiting loop up to dm_blk_ioctl so that it unlocks 
> when someone reloads the target?

I think it much more likely that multipathd will restore a path then an
entirely new table be loaded.
 
> BTW. there is also -EAGAIN dm_blk_ioctl if dm_suspended_md ... should this 
> -EAGAIN be removed too or not?

Don't think it needs to be removed.  Here is a patch that addresses the
issue for me.

---

From: Mike Snitzer <snitzer at redhat.com>
Subject: [PATCH] dm mpath: only retry ioctl if queue_if_no_path was configured

commit 35991652b ("dm mpath: allow ioctls to trigger pg init") should
have checked if queue_if_no_path was configured before queueing IO.

Checking for the queue_if_no_path feature, like is done in map_io(),
allows the following table load to work without blocking in the
multipath_ioctl retry loop:

  echo "0 1024 multipath 0 0 0 0" | dmsetup create mpath_nodevs

Without this fix the multipath_ioctl will block with the following stack
trace:

  blkid           D 0000000000000002     0 23936      1 0x00000000
   ffff8802b89e5cd8 0000000000000082 ffff8802b89e5fd8 0000000000012440
   ffff8802b89e4010 0000000000012440 0000000000012440 0000000000012440
   ffff8802b89e5fd8 0000000000012440 ffff88030c2aab30 ffff880325794040
  Call Trace:
   [<ffffffff814ce099>] schedule+0x29/0x70
   [<ffffffff814cc312>] schedule_timeout+0x182/0x2e0
   [<ffffffff8104dee0>] ? lock_timer_base+0x70/0x70
   [<ffffffff814cc48e>] schedule_timeout_uninterruptible+0x1e/0x20
   [<ffffffff8104f840>] msleep+0x20/0x30
   [<ffffffffa0000839>] multipath_ioctl+0x109/0x170 [dm_multipath]
   [<ffffffffa06bfb9c>] dm_blk_ioctl+0xbc/0xd0 [dm_mod]
   [<ffffffff8122a408>] __blkdev_driver_ioctl+0x28/0x30
   [<ffffffff8122a79e>] blkdev_ioctl+0xce/0x730
   [<ffffffff811970ac>] block_ioctl+0x3c/0x40
   [<ffffffff8117321c>] do_vfs_ioctl+0x8c/0x340
   [<ffffffff81166293>] ? sys_newfstat+0x33/0x40
   [<ffffffff81173571>] sys_ioctl+0xa1/0xb0
   [<ffffffff814d70a9>] system_call_fastpath+0x16/0x1b

Signed-off-by: Mike Snitzer <snitzer at redhat.com>
---
 drivers/md/dm-mpath.c |   11 +++++++----
 1 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
index d8abb90..034233e 100644
--- a/drivers/md/dm-mpath.c
+++ b/drivers/md/dm-mpath.c
@@ -1555,6 +1555,7 @@ static int multipath_ioctl(struct dm_target *ti, unsigned int cmd,
 			   unsigned long arg)
 {
 	struct multipath *m = ti->private;
+	struct pgpath *pgpath;
 	struct block_device *bdev;
 	fmode_t mode;
 	unsigned long flags;
@@ -1570,12 +1571,14 @@ again:
 	if (!m->current_pgpath)
 		__choose_pgpath(m, 0);
 
-	if (m->current_pgpath) {
-		bdev = m->current_pgpath->path.dev->bdev;
-		mode = m->current_pgpath->path.dev->mode;
+	pgpath = m->current_pgpath;
+
+	if (pgpath) {
+		bdev = pgpath->path.dev->bdev;
+		mode = pgpath->path.dev->mode;
 	}
 
-	if (m->queue_io)
+	if ((pgpath && m->queue_io) || (!pgpath && m->queue_if_no_path))
 		r = -EAGAIN;
 	else if (!bdev)
 		r = -EIO;
-- 
1.7.1