selftests/mm/hmm-tests: test pagemap reads of PMD device-private entries

To cover pagemap paths scanning PMD entries, add assertions to check
whether a device-private PMD entry has the correct pagemap information -
the PM_SWAP bit must be on in the pagemap entry.  Before that, we must
assert through HMM_DMIRROR_SNAPSHOT snapshot that the leaf entry is at PMD
level and not PTE level.

Link: https://lore.kernel.org/20260604055308.1947679-3-dev.jain@arm.com
Signed-off-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: David Hildenbrand (Arm) <david@kernel.org>
Cc: Oscar Salvador (SUSE) <osalvador@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Dev Jain

tools/testing/selftests/mm/hmm-tests.c

@@ -2274,8 +2274,11 @@ TEST_F(hmm, migrate_anon_huge_fault)
 	unsigned long npages;
 	unsigned long size;
 	unsigned long i;
+	unsigned char *m;
+	uint64_t entry;
 	void *old_ptr;
 	void *map;
+	int pagemap_fd;
 	int *ptr;
 	int ret;
 
@@ -2298,15 +2301,16 @@ TEST_F(hmm, migrate_anon_huge_fault)
 
 	npages = size >> self->page_shift;
 	map = (void *)ALIGN((uintptr_t)buffer->ptr, size);
-	ret = madvise(map, size, MADV_HUGEPAGE);
-	ASSERT_EQ(ret, 0);
 	old_ptr = buffer->ptr;
 	buffer->ptr = map;
 
 	/* Initialize buffer in system memory. */
 	for (i = 0, ptr = buffer->ptr; i < size / sizeof(*ptr); ++i)
 		ptr[i] = i;
 
+	ret = madvise(map, size, MADV_COLLAPSE);
+	ASSERT_EQ(ret, 0);
+
 	/* Migrate memory to device. */
 	ret = hmm_migrate_sys_to_dev(self->fd, buffer, npages);
 	ASSERT_EQ(ret, 0);
@@ -2316,6 +2320,32 @@ TEST_F(hmm, migrate_anon_huge_fault)
 	for (i = 0, ptr = buffer->mirror; i < size / sizeof(*ptr); ++i)
 		ASSERT_EQ(ptr[i], i);
 
+	if (!hmm_is_coherent_type(variant->device_number)) {
+		ret = hmm_dmirror_cmd(self->fd, HMM_DMIRROR_SNAPSHOT,
+				      buffer, npages);
+		ASSERT_EQ(ret, 0);
+		ASSERT_EQ(buffer->cpages, npages);
+
+		m = buffer->mirror;
+		for (i = 0; i < npages; ++i)
+			ASSERT_EQ(m[i], HMM_DMIRROR_PROT_DEV_PRIVATE_LOCAL |
+					HMM_DMIRROR_PROT_WRITE |
+					HMM_DMIRROR_PROT_PMD);
+
+		pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
+		ASSERT_GE(pagemap_fd, 0);
+
+		for (i = 0; i < npages; ++i) {
+			entry = pagemap_get_entry(pagemap_fd,
+					(char *)buffer->ptr + i * self->page_size);
+
+			ASSERT_NE(entry & PM_SWAP, 0);
+			ASSERT_FALSE(PAGEMAP_PRESENT(entry));
+		}
+
+		close(pagemap_fd);
+	}
+
 	/* Fault pages back to system memory and check them. */
 	for (i = 0, ptr = buffer->ptr; i < size / sizeof(*ptr); ++i)
 		ASSERT_EQ(ptr[i], i);