[dm-devel] multipath queues build invalid requests when all paths are lost

[Mikulas Patocka]

On Tue, 4 Sep 2012, Mike Snitzer wrote:

> On Tue, Sep 04 2012 at 12:10pm -0400,
> Mike Snitzer <snitzer at redhat.com> wrote:
> 
> > On Tue, Sep 04 2012 at 10:58am -0400,
> > Mike Snitzer <snitzer at redhat.com> wrote:
> > 
> > > On Fri, Aug 31 2012 at 11:04am -0400,
> > > David Jeffery <djeffery at redhat.com> wrote:
> > > 
> > > > 
> > > > The DM module recalculates queue limits based only on devices which currently
> > > > exist in the table.  This creates a problem in the event all devices are
> > > > temporarily removed such as all fibre channel paths being lost in multipath.
> > > > DM will reset the limits to the maximum permissible, which can then assemble
> > > > requests which exceed the limits of the paths when the paths are restored.  The
> > > > request will fail the blk_rq_check_limits() test when sent to a path with
> > > > lower limits, and will be retried without end by multipath.
> > > > 
> > > > This becomes a much bigger issue after fe86cdcef73ba19a2246a124f0ddbd19b14fb549.
> > > > Previously, most storage had max_sector limits which exceeded the default
> > > > value used.  This meant most setups wouldn't trigger this issue as the default
> > > > values used when there were no paths were still less than the limits of the
> > > > underlying devices.  Now that the default stacking values are no longer
> > > > constrained, any hardware setup can potentially hit this issue.
> > > > 
> > > > This proposed patch alters the DM limit behavior.  With the patch, DM queue
> > > > limits only go one way: more restrictive.  As paths are removed, the queue's
> > > > limits will maintain their current settings.  As paths are added, the queue's
> > > > limits may become more restrictive.
> > > 
> > > With your proposed patch you could still hit the problem if the
> > > initial multipath table load were to occur when no paths exist, e.g.:
> > > echo "0 1024 multipath 0 0 0 0" | dmsetup create mpath_nodevs 
> > > 
> > > (granted, this shouldn't ever happen.. as is evidenced by the fact
> > > that doing so will trigger an existing mpath bug; commit a490a07a67b
> > > "dm mpath: allow table load with no priority groups" clearly wasn't
> > > tested with the initial table load having no priority groups)
> > 
> > Hi Mikulas, 
> > 
> > It seems your new retry in multipath_ioctl (commit 3599165) is causing
> > problems for the above dmsetup create.
> > 
> > Here is the stack trace for a hang that resulted as a side-effect of
> > udev starting blkid for the newly created multipath device:
> > 
> > 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
> 
> FYI, here is the full blkid command line:
> /sbin/blkid -o udev -p /dev/dm-8

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?

BTW. there is also -EAGAIN dm_blk_ioctl if dm_suspended_md ... should this 
-EAGAIN be removed too or not?

Mikulas