On Wed, Nov 2, 2016 at 11:08 AM, Kent Overstreet <kent.overstreet@xxxxxxxxx> wrote: > On Mon, Oct 31, 2016 at 08:39:15AM -0700, Christoph Hellwig wrote: >> On Sat, Oct 29, 2016 at 04:08:27PM +0800, Ming Lei wrote: >> > Some drivers(such as dm) should be capable of dealing with multipage >> > bvec, but the incoming bio may be too big, such as, a new singlepage bvec >> > bio can't be cloned from the bio, or can't be allocated to singlepage >> > bvec with same size. >> > >> > At least crypt dm, log writes and bcache have this kind of issue. >> >> We already have the segment_size limitation for request based drivers. >> I'd rather extent it to bio drivers if really needed. >> >> But then again we should look into not having this limitation. E.g. >> for bcache I'd be really surprised if it's that limited, given that >> Kent came up with this whole multipage bvec scheme. > > AFAIK the only issue is with drivers that may have to bounce bios - pages that > were contiguous in the original bio won't necessarily be contiguous in the > bounced bio, thus bouncing might require more than BIO_MAX_SEGMENTS bvecs. > > I don't know what Ming's referring to by "singlepage bvec bios". > > Anyways, bouncing comes up in multiple places so we probably need to come up > with a generic solution for that. Other than that, there shouldn't be any issues > or limitations - if you're not bouncing, there's no need to clone the bvecs. AFAIK, the only special case is bch_data_verify(): drivers/md/bcache/debug.c, for other bio_clone(), no direct access to io vec table, so default multipage bvec copy is fine. I will remove the flag and try to fix bch_data_verify() by using multiple bio, and I remembered I cooked patch to do that long time ago, :-) Thanks, Ming Lei -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
