Messages in this thread | | | Date | Tue, 1 Sep 2020 06:34:26 +0100 | From | Christoph Hellwig <> | Subject | Re: [PATCH 04/11] block: Add bio_for_each_thp_segment_all |
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On Mon, Aug 31, 2020 at 08:48:37PM +0100, Matthew Wilcox wrote: > 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];
This should probably use bio_for_each_bvec_all instead of directly poking into the bio. I'd also be tempted to move the loop body into a separate helper, but that's just a slight stylistic preference.
> 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;
Shouldn't the page_size here be thp_size?
> 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.
Yes, that's exactly why I think avoiding bio_for_each_segment_all is a good idea in general.
> 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.
Drivers can absolutely see a bv_offset that is larger due to bio splitting. However the submitting file system should never see one unless it creates one, which would be stupid.
And yes, eventually bv_page and bv_offset should be replaced with a
phys_addr_t bv_phys;
and life would become simpler in many places (and the bvec would shrink for most common setups as well).
For now I'd end up with something like:
static void iomap_read_end_bvec(struct page *page, size_t offset, size_t length, int error) { while (length > 0) { size_t page_size = thp_size(page); size_t count = min(page_size - offset, length);
iomap_read_page_end_io(page, offset, count, error);
page += (offset + count) / page_size; length -= count; offset = 0; } }
static void iomap_read_end_io(struct bio *bio) { int i, error = blk_status_to_errno(bio->bi_status); struct bio_vec *bvec;
bio_for_each_bvec_all(bvec, bio, i) iomap_read_end_bvec(bvec->bv_page, bvec->bv_offset, bvec->bv_len, error; bio_put(bio); }
and maybe even merge iomap_read_page_end_io into iomap_read_end_bvec.
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