Re: [PATCH v2 3/5] mm/shmem: introduce copy_zero_to_iter() for large zeroing

[Matthew Wilcox <willy@infradead.org>]

On Mon, Jun 01, 2026 at 01:57:02PM +0800, Chi Zhiling wrote:
> Currently, holes larger than PAGE_SIZE cannot be handled because
> ZERO_PAGE is limited to a single page. Add copy_zero_to_iter() as a
> wrapper to support copying larger zero ranges to the iterator.

I think Hugh put this optimisation in the wrong place, and you're
perpetuating that ;-)

So perhaps we can start by moving this optimisation to lib/iov_iter.c?
And then you can redo your optimisation on top of that.

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 243662af1af7..06c54d719fcd 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -451,7 +451,23 @@ static __always_inline
 size_t zero_to_user_iter(void __user *iter_to, size_t progress,
 			 size_t len, void *priv, void *priv2)
 {
-	return clear_user(iter_to, len);
+	/*
+	 * it is noticeably faster to copy the zero page instead of
+	 * calling clear_user().  Shame.
+	 */
+	void *from = page_address(ZERO_PAGE(0));
+	size_t res = 0;
+
+	while (1) {
+		size_t n = min(len, PAGE_SIZE);
+		n = copy_to_user_iter(iter_to, progress, n, from, priv2);
+		res += n;
+		len -= n;
+		if (!len || !n)
+			break;
+	}
+
+	return res;
 }
 
 static __always_inline
diff --git a/mm/shmem.c b/mm/shmem.c
index 3b5dc21b323c..112cae9f9e4f 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3427,19 +3427,7 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
 			else
 				ret = copy_page_to_iter(page, offset, nr, to);
 			folio_put(folio);
-		} else if (user_backed_iter(to)) {
-			/*
-			 * Copy to user tends to be so well optimized, but
-			 * clear_user() not so much, that it is noticeably
-			 * faster to copy the zero page instead of clearing.
-			 */
-			ret = copy_page_to_iter(ZERO_PAGE(0), offset, nr, to);
 		} else {
-			/*
-			 * But submitting the same page twice in a row to
-			 * splice() - or others? - can result in confusion:
-			 * so don't attempt that optimization on pipes etc.
-			 */
 			ret = iov_iter_zero(nr, to);
 		}