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. NeilBrown
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