Re: NULL pointer due to malformed bcache bio

[Mike Snitzer <snitzer-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Wed, Apr 10 2013 at  6:49pm -0400,
Kent Overstreet <koverstreet-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> 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?