[dm-devel] linux-next dm-thinp regression due to "bio: skip atomic inc/dec of ->bi_remaining for non-chains"

[Mike Snitzer]

On Wed, May 13 2015 at  3:28pm -0400,
Mike Snitzer <snitzer at redhat.com> wrote:

> The device-mapper-test-suite (dmts) thinp tests no longer run due to
> linux-block.git commit c4cf5261 ("bio: skip atomic inc/dec of
> ->bi_remaining for non-chains")
> 
> That commit has been the focus of some other work I have pending because
> I was forced to adapt it due to bi_remaining no longer being accessible
> directly.  So it was surprising/interesting for this commit to take
> center stage in the context of dm-thinp regression.
> 
> bios don't get submitted (simplest reproducer is the 'dd_benchmark' test
> in the dmts thin-provisioning suite which just uses dd with oflag=direct
> immediately after the DM thin device is created).  The top-level DM
> device stays at 100% utilization but no progress is made.  Reverting
> the commit in question resolves the problem.
> 
> But I'm able to use lvm2 to create thin devices that allow IO to
> complete; so I haven't figured out what is so special about dmts (which
> uses dmsetup directly instead of lvm2).
> 
> I'll see if I can figure out what might be happening... but would
> welcome more eyes on this to see if anything stands out relative to the
> dm-thin.c changes.

I've added some tracing.  This issue is isolated to the bio-chain that
is used for dm-thinp's overwrite support -- and other code that follows
this bio-chain pattern (so all non-blk-core bio_inc_remaining() callers
in commit c4cf5261.

In dm-thin.c replace bi_end_io to point to overwrite_endio, at that time
__bi_remaining is 1 (but the new BIO_CHAIN flag is _not_ set):

 device-mapper: thin: save_and_set_endio: before overwrite sector=1536 bi_remaining=1

overwrite_endio() is called and the original end_io restored with
accompanying call to bio_inc_remaining():

 device-mapper: thin: overwrite_endio: before inc sector=1536 bi_remaining=1 
 device-mapper: thin: overwrite_endio:  after inc sector=1536 bi_remaining=2

Then bio_endio() is later called but the original endio won't get called
since __bi_remaining=2 (endio would only drop it to 1).

As you know commit c4cf5261 avoids the atomic operations during endio
unless BIO_CHAIN is set.  It only gets set if bio_inc_remaining() is
used.  But bio_inc_remaining() is only used _after_ the initial endio --
we needed BIO_CHAIN set earlier for these cases.  Seems we need a new
function to register the intent to cascade endios (to allow for proper
accounting)?

We could then switch callers to something like:

  bio_set_chain(bio); // establishes BIO_CHAIN iff __bio_remaining is 1?
  old = io->bi_end_io;
  bio->bi_end_io = new;

 in new():
  bio->bi_end_io = old;
  bio_inc_remaining(bio)

Anyway, not sure about the proper fix -- I do know this commit is a
definite regression (but it does nicely avoid the costly atomics ;)