tools/testing/selftests: fix guard region test tmpfs assumption

The current implementation of the guard region tests assume that /tmp is
mounted as tmpfs, that is shmem.

This isn't always the case, and at least one instance of a spurious test
failure has been reported as a result.

This assumption is unsafe, rushed and silly - and easily remedied by
simply using memfd, so do so.

We also have to fixup the readonly_file test to explicitly only be
applicable to file-backed cases.

Link: https://lkml.kernel.org/r/20250425162436.564002-1-lorenzo.stoakes@oracle.com
Fixes: 272f37d3e99a ("tools/selftests: expand all guard region tests to file-backed")
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reported-by: Ryan Roberts <ryan.roberts@arm.com>
Closes: https://lore.kernel.org/linux-mm/a2d2766b-0ab4-437b-951a-8595a7506fe9@arm.com/
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/tools/testing/selftests/mm/guard-regions.c b/tools/testing/selftests/mm/guard-regions.c
index b3d0e277109616710e5b1db0c0170651ae790a47..eba43ead13ae82aa0936c62424216256196d4921 100644 (file)
--- a/tools/testing/selftests/mm/guard-regions.c
+++ b/tools/testing/selftests/mm/guard-regions.c
@@ -271,12 +271,16 @@ FIXTURE_SETUP(guard_regions)
        self->page_size = (unsigned long)sysconf(_SC_PAGESIZE);
        setup_sighandler();
 
-       if (variant->backing == ANON_BACKED)
+       switch (variant->backing) {
+       case ANON_BACKED:
                return;
-
-       self->fd = open_file(
-               variant->backing == SHMEM_BACKED ? "/tmp/" : "",
-               self->path);
+       case LOCAL_FILE_BACKED:
+               self->fd = open_file("", self->path);
+               break;
+       case SHMEM_BACKED:
+               self->fd = memfd_create(self->path, 0);
+               break;
+       }
 
        /* We truncate file to at least 100 pages, tests can modify as needed. */
        ASSERT_EQ(ftruncate(self->fd, 100 * self->page_size), 0);
@@ -1696,7 +1700,7 @@ TEST_F(guard_regions, readonly_file)
        char *ptr;
        int i;
 
-       if (variant->backing == ANON_BACKED)
+       if (variant->backing != LOCAL_FILE_BACKED)
                SKIP(return, "Read-only test specific to file-backed");
 
        /* Map shared so we can populate with pattern, populate it, unmap. */