Re: [PATCH 04/11] block: Add bio_for_each_thp_segment_all

[Matthew Wilcox]

On Thu, Aug 27, 2020 at 09:44:31AM +0100, Christoph Hellwig wrote:
> On Mon, Aug 24, 2020 at 04:16:53PM +0100, Matthew Wilcox (Oracle) wrote:
> > Iterate once for each THP instead of once for each base page.
> 
> FYI, I've always been wondering if bio_for_each_segment_all is the
> right interface for the I/O completions, because we generally don't
> need the fake bvecs for each page.  Only the first page can have an
> offset, and only the last page can be end earlier than the end of
> the page size.
> 
> It would seem way more efficient to just have a helper that extracts
> the offset and end, and just use that in a loop that does the way
> cheaper iteration over the physical addresses only.  This might (or
> might) not be a good time to switch to that model for iomap.

Something like this?

static void iomap_read_end_io(struct bio *bio)
{
        int i, error = blk_status_to_errno(bio->bi_status);

        for (i = 0; i < bio->bi_vcnt; i++) {
                struct bio_vec *bvec = &bio->bi_io_vec[i];
                size_t offset = bvec->bv_offset;
                size_t length = bvec->bv_len;
                struct page *page = bvec->bv_page;

                while (length > 0) { 
                        size_t count = thp_size(page) - offset;
                        
                        if (count > length)
                                count = length;
                        iomap_read_page_end_io(page, offset, count, error);
                        page += (offset + count) / PAGE_SIZE;
                        length -= count;
                        offset = 0;
                }
        }

        bio_put(bio);
}

Maybe I'm missing something important here, but it's significantly
simpler code -- iomap_read_end_io() goes down from 816 bytes to 560 bytes
(256 bytes less!) iomap_read_page_end_io is inlined into it both before
and after.

There is some weirdness going on with regards to bv_offset that I don't
quite understand.  In the original bvec_advance:

                bv->bv_page = bvec->bv_page + (bvec->bv_offset >> PAGE_SHIFT);
                bv->bv_offset = bvec->bv_offset & ~PAGE_MASK;

which I cargo-culted into bvec_thp_advance as:

                bv->bv_page = thp_head(bvec->bv_page +
                                (bvec->bv_offset >> PAGE_SHIFT));
                page_size = thp_size(bv->bv_page);
                bv->bv_offset = bvec->bv_offset -
                                (bv->bv_page - bvec->bv_page) * PAGE_SIZE;

Is it possible to have a bvec with an offset that is larger than the
size of bv_page?  That doesn't seem like a useful thing to do, but
if that needs to be supported, then the code up top doesn't do that.
We maybe gain a little bit by counting length down to 0 instead of
counting it up to bv_len.  I dunno; reading the code over now, it
doesn't seem like that much of a difference.

Maybe you meant something different?