On 01/12/2020 13:32, Christoph Hellwig wrote:
> 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:
Yeah, that's the idea, but also wanted to verify that callers don't
free it while in use, or if that's not the case to make it conditional
by adding a flag in iov_iter.
Can anybody vow right off the bat that all callers behave well?
>
> 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;
> }
>
--
Pavel Begunkov
