Re: [RFC PATCH 2/2] dm: only initialize full request_queue forrequest-based device

[Mike Snitzer]

On Wed, May 19 2010 at  8:01am -0400,
Mike Snitzer <snitzer@xxxxxxxxxx> wrote:

> On Wed, May 19, 2010 at 1:57 AM, Kiyoshi Ueda <k-ueda@xxxxxxxxxxxxx> wrote:
> > Hi Mike,
> >
> > On 05/18/2010 10:46 PM +0900, Mike Snitzer wrote:
> >> I'll post v5 of the overall patch which will revert the mapped_device
> >> 'queue_lock' serialization that I proposed in v4.  v5 will contain
> >> the following patch to localize all table load related queue
> >> manipulation to the _hash_lock protected critical section in
> >> table_load().  So it sets the queue up _after_ the table's type is
> >> established with dm_table_set_type().
> >
> > dm_table_setup_md_queue() may allocate memory with blocking mode.
> > Blocking allocation inside exclusive _hash_lock can cause deadlock;
> > e.g. when it has to wait for other dm devices to resume to free some
> > memory.
> 
> We make no guarantees that other DM devices will resume before a table
> load -- so calling dm_table_setup_md_queue() within the exclusive
> _hash_lock is no different than other DM devices being suspended while
> a request-based DM device performs its first table_load().
> 
> My thinking was this should not be a problem as it is only valid to
> call dm_table_setup_md_queue() before the newly created request-based
> DM device has been resumed.
> 
> AFAIK we don't have any explicit constraints on memory allocations
> during table load (e.g. table loads shouldn't depend on other devices'
> writeback) -- but any GFP_KERNEL allocation could recurse
> (elevator_alloc() currently uses GFP_KERNEL with kmalloc_node)...
> 
> I'll have to review the DM code further to see if all memory
> allocations during table_load() are done via mempools.  I'll also
> bring this up on this week's LVM call.

We discussed this and I understand the scope of the problem now.

Just reiterating what you covered when you first pointed this issue out:

It could be that a table load gets blocked (waiting on a memory
allocation).  The table load can take as long as it needs.  But we can't
have it block holding the exclusive _hash_lock while blocking.  Having
_hash_lock prevents further DM ioctls.  The table load's allocation may
be blocking waiting for writeback to a DM device that will be resumed by
another thread.

Thanks again for pointing this out; I'll work to arrive at an
alternative locking scheme.  Likely introduce a lock local to the
multiple_device (effectively the 'queue_lock' I had before).  But
difference is I'd take that lock before taking _hash_lock.

I hope to have v6 available at some point today but it may be delayed by
a day.

> > Also, your patch changes the queue configuration even when a table is
> > already active and used.  (e.g. Loading bio-based table to a mapped_device
> > which is already active/used as request-based sets q->requst_fn in NULL.)
> > That could cause some critical problems.
> 
> Yes, that is possible and I can add additional checks to prevent this.
> But this speaks to a more general problem with the existing DM code.
> 
> dm_swap_table() has the negative check to prevent such table loads, e.g.:
> /* cannot change the device type, once a table is bound */
> 
> This check should come during table_load, as part of
> dm_table_set_type(), rather than during table resume.

I'll look to address this issue in v6 too.

Regards,
Mike
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