Re: [PATCH 28/60] block: introduce QUEUE_FLAG_SPLIT_MP
From: Ming Lei
Date: Thu Nov 03 2016 - 07:26:59 EST
On Thu, Nov 3, 2016 at 7:20 PM, Kent Overstreet
> 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.