[PATCH V8 07/33] iov_iter: ii_iovec_copy_to_user should pre-fault user pages

[Dave Kleikamp]

This duplicates the optimization in file_read_actor as a later patch
will replace it with a call to __iov_iter_copy_to_user().

Signed-off-by: Dave Kleikamp <dave.kleikamp@xxxxxxxxxx>
---
 fs/iov-iter.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/fs/iov-iter.c b/fs/iov-iter.c
index 6cb6be0..59f9556 100644
--- a/fs/iov-iter.c
+++ b/fs/iov-iter.c
@@ -80,17 +80,32 @@ static size_t ii_iovec_copy_to_user(struct page *page,
 			return 0;
 	}
 
-	kaddr = kmap(page);
 	if (likely(i->nr_segs == 1)) {
 		int left;
 		char __user *buf = iov->iov_base + i->iov_offset;
+		/*
+		 * Faults on the destination of a read are common, so do it
+		 * before taking the kmap.
+		 */
+		if (!fault_in_pages_writeable(buf, bytes)) {
+			kaddr = kmap_atomic(page);
+			left = __copy_to_user_inatomic(buf, kaddr + offset,
+						     bytes);
+			kunmap_atomic(kaddr);
+			if (left == 0)
+				goto success;
+		}
+		kaddr = kmap(page);
 		left = copy_to_user(buf, kaddr + offset, bytes);
+		kunmap(page);
+success:
 		copied = bytes - left;
 	} else {
+		kaddr = kmap(page);
 		copied = __iovec_copy_to_user(kaddr + offset, iov,
 					      i->iov_offset, bytes, 0);
+		kunmap(page);
 	}
-	kunmap(page);
 	return copied;
 }
 
-- 
1.8.3.4

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