Re: [PATCH] block: add bio_iov_iter_nvecs for figuring out nr_vecs

[Christoph Hellwig <hch@infradead.org>]

On Tue, Dec 01, 2020 at 01:17:49PM +0000, Pavel Begunkov wrote:
> I was thinking about memcpy bvec instead of iterating as a first step,
> and then try to reuse passed in bvec.
> 
> A thing that doesn't play nice with that is setting BIO_WORKINGSET in
> __bio_add_page(), which requires to iterate all pages anyway. I have no
> clue what it is, so rather to ask if we can optimise it out somehow?
> Apart from pre-computing for specific cases...
> 
> E.g. can pages of a single bvec segment be both in and out of a working
> set? (i.e. PageWorkingset(page)).

Adding Johannes for the PageWorkingset logic, which keeps confusing me
everytime I look at it.  I think it is intended to deal with pages
being swapped out and in, and doesn't make much sense to look at in
any form for direct I/O, but as said I'm rather confused by this code.

If PageWorkingset is a non-issue we should be able to just point the
bio at the biovec array.  I think that be done by allocating the bio
with nr_iovecs == 0, and then just updating >bi_io_vec and ->bi_vcnt
using a little helper like this:

static inline void bio_assign_bvec(struct bio *bio, struct bio_vec *bvecs,
		unsigned short nr_bvecs)
{
	WARN_ON_ONCE(BVEC_POOL_IDX(bio) != 0);
	bio->bi_io_vec = bvecs;
	bio->bi_vcnt = nr_bvecs;
}