[PATCH V10 15/19] block: always define BIO_MAX_PAGES as 256

[Huang, Ying]

Ming Lei <ming.lei at redhat.com> writes:

> On Thu, Nov 15, 2018 at 05:59:36PM -0800, Omar Sandoval wrote:
>> On Thu, Nov 15, 2018 at 04:53:02PM +0800, Ming Lei wrote:
>> > Now multi-page bvec can cover CONFIG_THP_SWAP, so we don't need to
>> > increase BIO_MAX_PAGES for it.
>> 
>> You mentioned to it in the cover letter, but this needs more explanation
>> in the commit message. Why did CONFIG_THP_SWAP require > 256? Why does
>> multipage bvecs remove that requirement?
>
> CONFIG_THP_SWAP needs to split one TH page into normal pages and adds
> them all to one bio. With multipage-bvec, it just takes one bvec to
> hold them all.

Yes.  CONFIG_THP_SWAP needs to put 512 normal sub-pages into one bio to
write the 512 sub-pages together.  With the help of multipage-bvec, it
needs just bvect to hold 512 normal sub-pages.

Best Regards,
Huang, Ying

> thanks,
> Ming