On 20/11/2020 02:22, Ming Lei wrote:
> On Fri, Nov 20, 2020 at 01:39:05AM +0000, Pavel Begunkov wrote:
>> On 20/11/2020 01:20, Matthew Wilcox wrote:
>>> On Thu, Nov 19, 2020 at 11:24:38PM +0000, Pavel Begunkov wrote:
>>>> The block layer spends quite a while in iov_iter_npages(), but for the
>>>> bvec case the number of pages is already known and stored in
>>>> iter->nr_segs, so it can be returned immediately as an optimisation
>>>
>>> Er ... no, it doesn't. nr_segs is the number of bvecs. Each bvec can
>>> store up to 4GB of contiguous physical memory.
>>
>> Ah, really, missed min() with PAGE_SIZE in bvec_iter_len(), then it's a
>> stupid statement. Thanks!
>>
>
> iov_iter_npages(bvec) still can be improved a bit by the following way:
Yep, was doing exactly that, +a couple of other places that are in my way.
>
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index 1635111c5bd2..d85ed7acce05 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -1608,17 +1608,23 @@ int iov_iter_npages(const struct iov_iter *i, int maxpages)
> npages = pipe_space_for_user(iter_head, pipe->tail, pipe);
> if (npages >= maxpages)
> return maxpages;
> + } else if (iov_iter_is_bvec(i)) {
> + unsigned idx, offset = i->iov_offset;
> +
> + for (idx = 0; idx < i->nr_segs; idx++) {
> + npages += DIV_ROUND_UP(i->bvec[idx].bv_len - offset,
> + PAGE_SIZE);
> + offset = 0;
> + }
> + if (npages >= maxpages)
> + return maxpages;
> } else iterate_all_kinds(i, size, v, ({
> unsigned long p = (unsigned long)v.iov_base;
> npages += DIV_ROUND_UP(p + v.iov_len, PAGE_SIZE)
> - p / PAGE_SIZE;
> if (npages >= maxpages)
> return maxpages;
> - 0;}),({
> - npages++;
> - if (npages >= maxpages)
> - return maxpages;
> - }),({
> + 0;}),0,({
> unsigned long p = (unsigned long)v.iov_base;
> npages += DIV_ROUND_UP(p + v.iov_len, PAGE_SIZE)
> - p / PAGE_SIZE;
>
--
Pavel Begunkov
