diff --git a/INSTALL b/INSTALL
index 5e22623b..46833a39 100644
--- a/INSTALL
+++ b/INSTALL
@@ -12,10 +12,11 @@ Dependencies:
  * libzstd
  * pkg-config
  * zlib1g
+ * valgrind
 
 On debian, you can install these with
     apt install -y pkg-config libaio-dev libblkid-dev libkeyutils-dev \
         liblz4-dev libscrypt-dev libsodium-dev liburcu-dev libzstd-dev \
-        uuid-dev zlib1g-dev
+        uuid-dev zlib1g-dev valgrind
 
 Then, just make && make install
diff --git a/linux/blkdev.c b/linux/blkdev.c
index 156d5353..8139e18d 100644
--- a/linux/blkdev.c
+++ b/linux/blkdev.c
@@ -10,6 +10,8 @@
 
 #include <libaio.h>
 
+#include <valgrind/memcheck.h>
+
 #include <linux/bio.h>
 #include <linux/blkdev.h>
 #include <linux/completion.h>
@@ -45,12 +47,20 @@ void generic_make_request(struct bio *bio)
 	iov = alloca(sizeof(*iov) * i);
 
 	i = 0;
-	bio_for_each_segment(bv, bio, iter)
+	bio_for_each_segment(bv, bio, iter) {
+		void *start = page_address(bv.bv_page) + bv.bv_offset;
+		size_t len = bv.bv_len;
+
 		iov[i++] = (struct iovec) {
-			.iov_base = page_address(bv.bv_page) + bv.bv_offset,
-			.iov_len = bv.bv_len,
+			.iov_base = start,
+			.iov_len = len,
 		};
 
+		/* To be pedantic it should only be on IO completion. */
+		if (bio_op(bio) == REQ_OP_READ)
+			VALGRIND_MAKE_MEM_DEFINED(start, len);
+	}
+
 	struct iocb iocb = {
 		.data		= bio,
 		.aio_fildes	= bio->bi_opf & REQ_FUA