[PATCH v4 3/4] iov_iter: add copy_page_to_iter_atomic()

[Lorenzo Stoakes]

Provide an atomic context equivalent for copy_page_to_iter(). This eschews
the might_fault() check copies memory in the same way that
copy_page_from_iter_atomic() does.

This functions assumes a non-compound page, however this mimics the
existing behaviour of copy_page_from_iter_atomic(). I am keeping the
behaviour consistent between the two, deferring any such change to an
explicit folio-fication effort.

This is being added in order that an iteratable form of vread() can be
implemented with known prefaulted pages to avoid the need for mutex
locking.

Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
---
 include/linux/uio.h |  2 ++
 lib/iov_iter.c      | 28 ++++++++++++++++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/include/linux/uio.h b/include/linux/uio.h
index 27e3fd942960..fab07103090f 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -154,6 +154,8 @@ static inline struct iovec iov_iter_iovec(const struct iov_iter *iter)
 
 size_t copy_page_from_iter_atomic(struct page *page, unsigned offset,
 				  size_t bytes, struct iov_iter *i);
+size_t copy_page_to_iter_atomic(struct page *page, unsigned offset,
+				size_t bytes, struct iov_iter *i);
 void iov_iter_advance(struct iov_iter *i, size_t bytes);
 void iov_iter_revert(struct iov_iter *i, size_t bytes);
 size_t fault_in_iov_iter_readable(const struct iov_iter *i, size_t bytes);
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 274014e4eafe..48ca1c5dfc04 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -821,6 +821,34 @@ size_t copy_page_from_iter_atomic(struct page *page, unsigned offset, size_t byt
 }
 EXPORT_SYMBOL(copy_page_from_iter_atomic);
 
+size_t copy_page_to_iter_atomic(struct page *page, unsigned offset, size_t bytes,
+				struct iov_iter *i)
+{
+	char *kaddr = kmap_local_page(page);
+	char *p = kaddr + offset;
+	size_t copied = 0;
+
+	if (!page_copy_sane(page, offset, bytes) ||
+	    WARN_ON_ONCE(i->data_source))
+		goto out;
+
+	if (unlikely(iov_iter_is_pipe(i))) {
+		copied = copy_page_to_iter_pipe(page, offset, bytes, i);
+		goto out;
+	}
+
+	iterate_and_advance(i, bytes, base, len, off,
+		copyout(base, p + off, len),
+		memcpy(base, p + off, len)
+	)
+	copied = bytes;
+
+out:
+	kunmap_local(kaddr);
+	return copied;
+}
+EXPORT_SYMBOL(copy_page_to_iter_atomic);
+
 static void pipe_advance(struct iov_iter *i, size_t size)
 {
 	struct pipe_inode_info *pipe = i->pipe;
-- 
2.39.2