linux-kernel - Re: [PATCH v2 1/2] iov_iter: optimise iov_iter

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Date:   Fri, 20 Nov 2020 10:22:00 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        linux-fsdevel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] iov_iter: optimise iov_iter_npages for bvec

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