[RFC,net-next 3/6] iov_iter: Add a nocache copy iov iterator

[Joe Damato <jdamato@fastly.com>]

Add copy_page_from_iter_nocache, which wraps copy_page_from_iter_iovec and
passes in a custom copyin function: __copy_from_user_nocache.

This allows callers of copy_page_from_iter_nocache to copy data without
disturbing the CPU cache.

Signed-off-by: Joe Damato <jdamato@fastly.com>
---
 include/linux/uio.h |  2 ++
 lib/iov_iter.c      | 20 ++++++++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/include/linux/uio.h b/include/linux/uio.h
index 739285f..58c7946 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -142,6 +142,8 @@ size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
 			 struct iov_iter *i);
 size_t copy_page_from_iter(struct page *page, size_t offset, size_t bytes,
 			 struct iov_iter *i);
+size_t copy_page_from_iter_nocache(struct page *page, size_t offset, size_t bytes,
+			 struct iov_iter *i);
 
 size_t _copy_to_iter(const void *addr, size_t bytes, struct iov_iter *i);
 size_t _copy_from_iter(void *addr, size_t bytes, struct iov_iter *i);
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index ef22ec1..985bf58 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -895,6 +895,26 @@ size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
 }
 EXPORT_SYMBOL(copy_page_to_iter);
 
+size_t copy_page_from_iter_nocache(struct page *page, size_t offset, size_t
+		bytes, struct iov_iter *i)
+{
+	if (unlikely(!page_copy_sane(page, offset, bytes)))
+		return 0;
+	if (unlikely(iov_iter_is_pipe(i) || iov_iter_is_discard(i))) {
+		WARN_ON(1);
+		return 0;
+	}
+	if (iov_iter_is_bvec(i) || iov_iter_is_kvec(i) || iov_iter_is_xarray(i)) {
+		void *kaddr = kmap_atomic(page);
+		size_t wanted = _copy_from_iter_nocache(kaddr + offset, bytes, i);
+
+		kunmap_atomic(kaddr);
+		return wanted;
+	} else
+		return copy_page_from_iter_iovec(page, offset, bytes, i,
+				__copy_from_user_nocache);
+}
+
 size_t copy_page_from_iter(struct page *page, size_t offset, size_t bytes,
 			 struct iov_iter *i)
 {
-- 
2.7.4