diff --git a/reftable/blocksource.c b/reftable/blocksource.c
index 573c81287f..7f7441f751 100644
--- a/reftable/blocksource.c
+++ b/reftable/blocksource.c
@@ -93,13 +93,12 @@ void block_source_from_buf(struct reftable_block_source *bs,
 }
 
 struct file_block_source {
-	uint64_t size;
-	unsigned char *data;
+	struct reftable_mmap mmap;
 };
 
 static uint64_t file_size(void *b)
 {
-	return ((struct file_block_source *)b)->size;
+	return ((struct file_block_source *)b)->mmap.size;
 }
 
 static void file_release_data(void *b REFTABLE_UNUSED, struct reftable_block_data *dest REFTABLE_UNUSED)
@@ -109,7 +108,7 @@ static void file_release_data(void *b REFTABLE_UNUSED, struct reftable_block_dat
 static void file_close(void *v)
 {
 	struct file_block_source *b = v;
-	munmap(b->data, b->size);
+	reftable_munmap(&b->mmap);
 	reftable_free(b);
 }
 
@@ -117,8 +116,8 @@ static ssize_t file_read_data(void *v, struct reftable_block_data *dest, uint64_
 			      uint32_t size)
 {
 	struct file_block_source *b = v;
-	assert(off + size <= b->size);
-	dest->data = b->data + off;
+	assert(off + size <= b->mmap.size);
+	dest->data = (unsigned char *) b->mmap.data + off;
 	dest->len = size;
 	return size;
 }
@@ -156,13 +155,9 @@ int reftable_block_source_from_file(struct reftable_block_source *bs,
 		goto out;
 	}
 
-	p->size = st.st_size;
-	p->data = mmap(NULL, st.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
-	if (p->data == MAP_FAILED) {
-		err = REFTABLE_IO_ERROR;
-		p->data = NULL;
+	err = reftable_mmap(&p->mmap, fd, st.st_size);
+	if (err < 0)
 		goto out;
-	}
 
 	assert(!bs->ops);
 	bs->ops = &file_vtable;
diff --git a/reftable/system.c b/reftable/system.c
index cd76e56be8..9063641f30 100644
--- a/reftable/system.c
+++ b/reftable/system.c
@@ -143,3 +143,23 @@ uint64_t reftable_time_ms(void)
 {
 	return getnanotime() / 1000000;
 }
+
+int reftable_mmap(struct reftable_mmap *out, int fd, size_t len)
+{
+	void *data = xmmap_gently(NULL, len, PROT_READ, MAP_PRIVATE, fd, 0);
+	if (data == MAP_FAILED)
+		return REFTABLE_IO_ERROR;
+
+	out->data = data;
+	out->size = len;
+
+	return 0;
+}
+
+int reftable_munmap(struct reftable_mmap *mmap)
+{
+	if (munmap(mmap->data, mmap->size) < 0)
+		return REFTABLE_IO_ERROR;
+	memset(mmap, 0, sizeof(*mmap));
+	return 0;
+}
diff --git a/reftable/system.h b/reftable/system.h
index 071bfa3d58..c0e2cbe0ff 100644
--- a/reftable/system.h
+++ b/reftable/system.h
@@ -114,4 +114,22 @@ int flock_commit(struct reftable_flock *l);
 /* Report the time in milliseconds. */
 uint64_t reftable_time_ms(void);
 
+struct reftable_mmap {
+       void *data;
+       size_t size;
+       void *priv;
+};
+
+/*
+ * Map the file into memory. Returns 0 on success, a reftable error code on
+ * error.
+ */
+int reftable_mmap(struct reftable_mmap *out, int fd, size_t len);
+
+/*
+ * Unmap the file from memory. Returns 0 on success, a reftable error code on
+ * error.
+ */
+int reftable_munmap(struct reftable_mmap *mmap);
+
 #endif