Re: [RFC] using mempools for raid5-cache

On Wed, Dec 09, 2015 at 11:36:30AM +1100, NeilBrown wrote:

On Thu, Dec 03 2015, Christoph Hellwig wrote:

quoted

Currently the raid5-cache code is heavily relying on GFP_NOFAIL allocations.

I've looked into replacing these with mempools and biosets, and for the
bio and the meta_page that's pretty trivial as they have short life times
and do make guaranteed progress.  I'm massively struggling with the iounit
allocation, though.  These can live on for a long time over log I/O, cache
flushing and last but not least RAID I/O, and every attempt at something
mempool-like results in reproducible deadlocks.  I wonder if we need to
figure out some more efficient data structure to communicate the completion
status that doesn't rely on these fairly long living allocations from
the I/O path.

Presumably the root cause of these deadlocks is that the raid5d thread
has called
   handle_stripe -> ops_run_io ->r5l_write_stripe -> r5l_log_stripe
      -> r5l_get_meta -> r5l_new_meta

and r5l_new_meta is blocked on memory allocation, which won't complete
until some raid5 stripes get written out, which requires raid5d to do
something more useful than sitting and waiting.

I suspect a good direction towards a solution would be to allow the
memory allocation to fail, to cleanly propagate that failure indication
up through r5l_log_stripe to r5l_write_stripe which falls back to adding
the stripe_head to ->no_space_stripes.

Then we only release stripes from no_space_stripes when a memory
allocation might succeed.

There are lots of missing details, and possibly we would need a separate
list rather than re-using no_space_stripes.
But the key idea is that raid5d should never block (except beneath
submit_bio on some other device) and when it cannot make progress
without blocking, it should queue the stripe_head for later handling.

Does that make sense?

It does remove the scary __GFP_NOFAIL, but the approach is essentially
idential to a 'retry after allocation failure'. Why not just let the mm
(with __GFP_NOFAIL) to do the retry then?

Thanks,
Shaohua