Re: [PATCH 2/8] io_uring: add infra for importing vectored reg buffers

[Pavel Begunkov <asml.silence@gmail.com>]

On 3/3/25 20:49, Caleb Sander Mateos wrote:
> On Mon, Mar 3, 2025 at 7:51 AM Pavel Begunkov <asml.silence@gmail.com> wrote:
...
> 
> If I understand correctly, io_import_reg_vec() converts the iovecs to
> bio_vecs in place. If an iovec expands to more than one bio_vec (i.e.
> crosses a folio boundary), wouldn't the bio_vecs overwrite iovecs that
> hadn't been processed yet?

It's handled, obviously, you missed that the vectors are
offset'ed from each other.

> >> diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c
>> index 9b05e614819e..1ec1f5b3e385 100644
>> --- a/io_uring/rsrc.c
>> +++ b/io_uring/rsrc.c
...
>> +               for (; iov_len; offset = 0, bvec_idx++, src_bvec++) {
>> +                       size_t seg_size = min_t(size_t, iov_len,
>> +                                               folio_size - offset);
>> +
>> +                       res_bvec[bvec_idx].bv_page = src_bvec->bv_page;
>> +                       res_bvec[bvec_idx].bv_offset = offset;
>> +                       res_bvec[bvec_idx].bv_len = seg_size;
> 
> Could just increment res_bvec to avoid the variable bvec_idx?

I don't see the benefit.

>> +       for (i = 0; i < nr_iovs; i++)
>> +               max_segs += (iov[i].iov_len >> shift) + 2;
> 
> Sees like this may overestimate a bit. I think something like this
> would give the exact number of segments for each iovec?
> (((u64)iov_base & folio_mask) + iov_len + folio_mask) >> folio_shift

It's overestimated exactly to avoid a beast like this.

-- 
Pavel Begunkov