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Re: [dm-devel] [PATCHES] convert dm-thin to use dm-bufio


On Fri, Aug 19, 2011 at 10:12:24AM +0100, Joe Thornber wrote:
> On Thu, Aug 18, 2011 at 06:31:13PM -0400, Mikulas Patocka wrote:
> > I uploaded bufio-based block manager at 
> > http://people.redhat.com/mpatocka/patches/kernel/dm-thinp-bufio/. It 
> > supports locks, but it defines new functions down_write_non_owner and 
> > up_write_non_owner.
> Thanks Mikulas, I'll try and get it merged over the next couple of days.

I've merged and pushed this to my git repo.

All the test-suite is passing.  Performance is identical (though
hopefully we're a lot more scaleable now).

I have to say I think the separation of io and locking is very elegant
now.  Well done.

I've made two small changes:

i) I switched to using dm_bufio_new() rather than dm_bufio_read() at
the start of *write_lock_zero().  No point reading the data if we're
going to throw it away.

ii) We no longer hold the superblock lock for the duration of the
transaction.  This means we can get rid of the nasty rwsem non-owner

We have one outstanding issue however.  Now we're using a rwsem per
block, lockdep is trying to validate there are no deadlocks.  Which it
can't do, so is producing a warning. See the
 "Exception: Nested data dependencies leading to nested locking"

section in lockdep-design.txt for more discussion of the problem.

We need to suppress this somehow.  I guess the right thing to do is
use the nesting level facility, eg call the superblock level 0, btree
roots level 1 etc.  This will involve a lot of code changes however.
So for now I just want to turn off checking on those locks.

- Joe

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