From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mike Snitzer Subject: Re: NULL pointer due to malformed bcache bio Date: Wed, 10 Apr 2013 20:03:42 -0400 Message-ID: <20130411000342.GA19451@redhat.com> References: <20130410205439.GA18092@redhat.com> <20130410224914.GD30871@google.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20130410224914.GD30871-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> Sender: linux-bcache-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Kent Overstreet Cc: linux-bcache-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, dm-devel-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, axboe-tSWWG44O7X1aa/9Udqfwiw@public.gmane.org List-Id: linux-bcache@vger.kernel.org On Wed, Apr 10 2013 at 6:49pm -0400, Kent Overstreet 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. > 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?