[PATCH 3/5] userfaultfd/selftests: create alias mappings in the shmem test

[Axel Rasmussen <axelrasmussen@google.com>]

Previously, we just allocated two shm areas: area_src and area_dst. With
this commit, change this so we also allocate area_src_alias, and
area_dst_alias.

area_*_alias and area_* (respectively) point to the same underlying
physical pages, but are different VMAs. In a future commit in this
series, we'll leverage this setup to exercise minor fault handling
support for shmem, just like we do in the hugetlb_shared test.

Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
---
 tools/testing/selftests/vm/userfaultfd.c | 29 +++++++++++++++++++++---
 1 file changed, 26 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/vm/userfaultfd.c b/tools/testing/selftests/vm/userfaultfd.c
index 859398efb4fe..4a18590fe0f8 100644
--- a/tools/testing/selftests/vm/userfaultfd.c
+++ b/tools/testing/selftests/vm/userfaultfd.c
@@ -298,8 +298,9 @@ static int shmem_release_pages(char *rel_area)
 
 static void shmem_allocate_area(void **alloc_area)
 {
-	unsigned long offset =
-		alloc_area == (void **)&area_src ? 0 : nr_pages * page_size;
+	void *area_alias = NULL;
+	bool is_src = alloc_area == (void **)&area_src;
+	unsigned long offset = is_src ? 0 : nr_pages * page_size;
 
 	*alloc_area = mmap(NULL, nr_pages * page_size, PROT_READ | PROT_WRITE,
 			   MAP_SHARED, shm_fd, offset);
@@ -308,12 +309,34 @@ static void shmem_allocate_area(void **alloc_area)
 		goto fail;
 	}
 
+	area_alias = mmap(NULL, nr_pages * page_size, PROT_READ | PROT_WRITE,
+			  MAP_SHARED, shm_fd, offset);
+	if (area_alias == MAP_FAILED) {
+		perror("mmap of memfd alias failed");
+		goto fail_munmap;
+	}
+
+	if (is_src)
+		area_src_alias = area_alias;
+	else
+		area_dst_alias = area_alias;
+
 	return;
 
+fail_munmap:
+	if (munmap(*alloc_area, nr_pages * page_size) < 0) {
+		perror("munmap of memfd failed\n");
+		exit(1);
+	}
 fail:
 	*alloc_area = NULL;
 }
 
+static void shmem_alias_mapping(__u64 *start, size_t len, unsigned long offset)
+{
+	*start = (unsigned long)area_dst_alias + offset;
+}
+
 struct uffd_test_ops {
 	unsigned long expected_ioctls;
 	void (*allocate_area)(void **alloc_area);
@@ -341,7 +364,7 @@ static struct uffd_test_ops shmem_uffd_test_ops = {
 	.expected_ioctls = SHMEM_EXPECTED_IOCTLS,
 	.allocate_area	= shmem_allocate_area,
 	.release_pages	= shmem_release_pages,
-	.alias_mapping = noop_alias_mapping,
+	.alias_mapping = shmem_alias_mapping,
 };
 
 static struct uffd_test_ops hugetlb_uffd_test_ops = {
-- 
2.30.0.617.g56c4b15f3c-goog