On Wed, Dec 09, 2015 at 05:34:45PM +1100, NeilBrown wrote: > On Wed, Dec 09 2015, Shaohua Li wrote: > > > On Wed, Dec 09, 2015 at 11:36:30AM +1100, NeilBrown wrote: > >> On Thu, Dec 03 2015, Christoph Hellwig wrote: > >> > >> > 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? > > > > Because deadlocks. > > If raid5d is waiting for the mm to allocated memory, then it cannot > retire write requests which could free up memory. Ok, I missed the deadlock issue. Looks raid5d can't sleep waitting for memory. That means the mempool for metadata page/bio doesn't work too, as raid5d might block and not dispatch previous IO. Your proposal looks reasonable. Would keeping NOFAIL and having a separate thread got r5l_log_stripe work too? Thanks, Shaohua -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
