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SubjectRe: [PATCH] iov_iter: optimise bvec iov_iter_advance()
> @@ -1077,6 +1077,20 @@ void iov_iter_advance(struct iov_iter *i, size_t size)
> i->count -= size;
> return;
> }
> + if (iov_iter_is_bvec(i)) {
> + struct bvec_iter bi;
> +
> + bi.bi_size = i->count;
> + bi.bi_bvec_done = i->iov_offset;
> + bi.bi_idx = 0;
> + bvec_iter_advance(i->bvec, &bi, size);
> +
> + i->bvec += bi.bi_idx;
> + i->nr_segs -= bi.bi_idx;
> + i->count = bi.bi_size;
> + i->iov_offset = bi.bi_bvec_done;
> + return;
> + }
> iterate_and_advance(i, size, v, 0, 0, 0)

I like the idea, but whu not avoid the on-stack bvec_iter and just
open code this entirely using a new helper? E.g.

static void bio_iov_iter_advance(struct iov_iter *i, size_t bytes)
{
unsigned int cnt;

i->count -= bytes;

bytes += i->iov_offset;
for (cnt = 0; bytes && bytes >= i->bvec[cnt].bv_len; cnt++)
bytes -= i->bvec[cnt].bv_len;
i->iov_offset = bytes;

i->bvec += cnt;
i->nr_segs -= cnt;
}

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