On Wed, Apr 10, 2013 at 08:03:42PM -0400, Mike Snitzer wrote: > On Wed, Apr 10 2013 at 6:49pm -0400, > Kent Overstreet <koverstreet@xxxxxxxxxx> wrote: > > > On Wed, Apr 10, 2013 at 04:54:40PM -0400, Mike Snitzer wrote: > > > Hey, > > > > > > So DM core clearly needs to be more defensive about the possibility for > > > a NULL return from bio_alloc_bioset() given I'm hitting a NULL pointer > > > in DM's alloc_tio() because nr_iovecs=512. bio_alloc_bioset()'s call to > > > bvec_alloc() only supports nr_iovecs up to BIO_MAX_PAGES (256). > > > > > > Seems bcache should be using bio_get_nr_vecs() or something else? > > > > > > But by using a bcache bucket size of 2MB, with the bcache staged in > > > Jens' for-next, I've caused bcache to issue bios with nr_iovecs=512: > > > > Argh. Why is dm using bi_max_vecs instead of bi_vcnt? I could hack > > around this in bcache but I think dm is doing the wrong thing here. > > But even bio_alloc_bioset() sets: bio->bi_max_vecs = nr_iovecs; > And bio_clone_bioset() calls bio_alloc_bioset() with bio->bi_max_vecs. > Similarly, __bio_clone() is using bi_max_vecs when cloning the bi_io_vec. > So I'm missing why DM is doing the wrong thing. I forgot about the bio_clone() one - you're right, that's also a problem. So, I had a patch queued up at one point as part of the immutable biovecs series that changed bio_clone() and the dm bio cloning/splitting stuff to use bio_segments() instead of bi_max_vecs. That is IMO a better way of doing it anyways and as far as I could tell perfectly safe (it was tested), but the patch ended up squashed for various reasons and I'm not sure I want to recreate it just for this... though it would be the cleanest fix. > > Unless I've missed something in my testing (and bcache's BIO_MAX_PAGES > > check isn't quite right, actually) bcache _is_ splitting its bios > > whenever bio_segments(bio) > BIO_MAX_PAGES, it's only bi_max_vecs that's > > potentially > BIO_MAX_PAGES. > > OK, but why drive bi_max_vecs larger than BIO_MAX_PAGES? bcache has a mempool for bios that are used for reading/writing (potentially) entire buckets - but in the case where we're only writing to part of a btree node and the bio didn't have to be split, that's when we pass down our original huge bio. I just had the horrible thought that an easy fix would probably be to just reset bi_max_vecs to bi_vcnt in bcache before passing it down. If I can't come up with any reasons that won't work, I may just do that. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
