[Drbd-dev] [PATCH v4 01/11] block: make generic_make_request handle arbitrarily sized bios

[Ming Lin]

On Tue, May 26, 2015 at 7:36 AM, Mike Snitzer <snitzer at redhat.com> wrote:
> On Fri, May 22 2015 at  2:18pm -0400,
> Ming Lin <mlin at kernel.org> wrote:
>
>> From: Kent Overstreet <kent.overstreet at gmail.com>
>>
>> The way the block layer is currently written, it goes to great lengths
>> to avoid having to split bios; upper layer code (such as bio_add_page())
>> checks what the underlying device can handle and tries to always create
>> bios that don't need to be split.
>>
>> But this approach becomes unwieldy and eventually breaks down with
>> stacked devices and devices with dynamic limits, and it adds a lot of
>> complexity. If the block layer could split bios as needed, we could
>> eliminate a lot of complexity elsewhere - particularly in stacked
>> drivers. Code that creates bios can then create whatever size bios are
>> convenient, and more importantly stacked drivers don't have to deal with
>> both their own bio size limitations and the limitations of the
>> (potentially multiple) devices underneath them.  In the future this will
>> let us delete merge_bvec_fn and a bunch of other code.
>
> This series doesn't take any steps to train upper layers
> (e.g. filesystems) to size their bios larger (which is defined as
> "whatever size bios are convenient" above).
>
> bio_add_page(), and merge_bvec_fn, served as the means for upper layers
> (and direct IO) to build up optimally sized bios.  Without a replacement
> (that I can see anyway) how is this patchset making forward progress
> (getting Acks, etc)!?
>
> I like the idea of reduced complexity associated with these late bio
> splitting changes I'm just not seeing how this is ready given there are
> no upper layer changes that speak to building larger bios..
>
> What am I missing?

See: [PATCH v4 02/11] block: simplify bio_add_page()
https://lkml.org/lkml/2015/5/22/754

Now bio_add_page() can build lager bios.
And blk_queue_split() can split the bios in ->make_request() if needed.

Thanks.

>
> Please advise, thanks!
> Mike