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:
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;
--
Ming
