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SubjectRe: [PATCH 28/60] block: introduce QUEUE_FLAG_SPLIT_MP
On Wed, Nov 2, 2016 at 11:08 AM, Kent Overstreet
<kent.overstreet@gmail.com> 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

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