On Thu, Nov 03, 2016 at 07:26:52PM +0800, Ming Lei wrote: > On Thu, Nov 3, 2016 at 7:20 PM, Kent Overstreet > <kent.overstreet@xxxxxxxxx> wrote: > > On Thu, Nov 03, 2016 at 06:38:57PM +0800, Ming Lei wrote: > >> 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, :-) > > > > You can #ifdef out the bch_data_verify() code, it's debug code that hasn't been > > used in ages. > > Though you didn't test it ages, it is sitll working in my last test, :-) > > But someone can enable that for debug too, I don't want to make him/her sad. Up to you :) It's not useful for anything but debugging though, so I wouldn't worry about impacting end users. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
