On Tue, 2023-11-07 at 23:28 -0800, Christoph Hellwig wrote:
> you have verified the sender or the content.
> On Tue, Nov 07, 2023 at 06:01:40PM +0800, Ming Lei wrote:
> > In case of big chunk sequential IO, bio's size is often not aligned
> with
> > this queue's max request size because of multipage bvec, then small
> sized
> > bio can be made by bio split, so try to align bio with max io size
> if
> > it isn't the last one.
>
> I have a hard time parsing this long sentence.
>
> > +/*
> > + * If we still have data and bio is full, this bio size may not be
> > + * aligned with max io size, small bio can be caused by split, try
> > + * to avoid this situation by aligning bio with max io size.
> > + *
> > + * Big chunk of sequential IO workload can benefit from this way.
> > + */
> > +if (!ret && iov_iter_count(iter) && bio->bi_bdev &&
> > +bio_op(bio) != REQ_OP_ZONE_APPEND) {
> > +unsigned trim = bio_align_with_io_size(bio);
>
> Besides the fact that bi_bdev should always be set, this really does
> thing backwards. Instead of rewding things which is really
> expensive don't add it in the first place.
>
We cannot predict the number of pages that can be filled in a bio as it
depends on the physical layout of memory buffer. The only option is to
limit the bio to the queue limit.
Even if I fill the bio with a large amount of pages, it will still be
split based on the queue limit at block layer. Therefore, I believe it
is appropriate to limit the bio size in this case.
We can enforce this limitation before extracting the page, which would
elimiate the need for iov_iter_revert.
--
Best Regards,
Ed Tsai
