On Fri, Dec 11 2015, Shaohua Li wrote: > On Fri, Dec 11, 2015 at 11:09:58AM +1100, NeilBrown wrote: >> On Fri, Dec 11 2015, Shaohua Li wrote: >> >> > 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. >> >> raid5d mustn't sleep waiting for something that only raid5d can provide. >> >> A bio allocated from a mempool will be returned to that mempool when the >> IO request completes (if the end_io routine calls bio_put, which it >> does). >> So when waiting on a mempool for a bio, you are only waiting for some >> underlying device to perform IO, you are not waiting for anything that >> raid5d might do. So that is safe. >> >> Similarly the meta_page is freed in the end_io function, so it is safe >> to wait for that. >> Note that the important feature of a mempool is not so much that it >> pre-allocates some memory. The important point is that when that >> pre-allocated memory is freed it can *only* be used by the owner of the >> mempool. >> By contrast memory allocated with NOFAIL, when freed might be used by >> any other caller anywhere in the kernel. >> So waiting on a mempool means waiting for certain well defined events. >> Waiting in NOFAIL could end up waiting for almost anything. So it is >> much harder to reason about deadlocks. >> >> The bio and the meta_page contrast with the r5l_io_unit which isn't >> freed by the first end_io, but remains until raid5d has done some more >> work. So raid5d cannot wait for a r5l_io_unit to be freed. > > I'm confused 2 bios are already enough to avoid deadlock, sorry. Can you > peek Christoph's patches? I'll work on a patch for r5l_io_unit. > I don't understand what you are tring to say. Yes, a bio pool of 2 bios is (I believe) enough to avoid a deadlock when allocation a bio. It no affect on any deadlock involved in allocating an r5l_io_unit. I have looked at Christoph's patches. What in particular did you hope I would see? NeilBrown
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