selftests/mm: fix incorrect buffer->mirror size in hmm2 double_map test

The hmm2 double_map test was failing due to an incorrect buffer->mirror
size.  The buffer->mirror size was 6, while buffer->ptr size was 6 *
PAGE_SIZE.  The test failed because the kernel's copy_to_user function was
attempting to copy a 6 * PAGE_SIZE buffer to buffer->mirror.  Since the
size of buffer->mirror was incorrect, copy_to_user failed.

This patch corrects the buffer->mirror size to 6 * PAGE_SIZE.

Test Result without this patch
==============================
 #  RUN           hmm2.hmm2_device_private.double_map ...
 # hmm-tests.c:1680:double_map:Expected ret (-14) == 0 (0)
 # double_map: Test terminated by assertion
 #          FAIL  hmm2.hmm2_device_private.double_map
 not ok 53 hmm2.hmm2_device_private.double_map

Test Result with this patch
===========================
 #  RUN           hmm2.hmm2_device_private.double_map ...
 #            OK  hmm2.hmm2_device_private.double_map
 ok 53 hmm2.hmm2_device_private.double_map

Link: https://lkml.kernel.org/r/[email protected]
Fixes: fee9f6d ("mm/hmm/test: add selftests for HMM")
Signed-off-by: Donet Tom <[email protected]>
Reviewed-by: Muhammad Usama Anjum <[email protected]>
Cc: Jérôme Glisse <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Mark Brown <[email protected]>
Cc: Przemek Kitszel <[email protected]>
Cc: Ritesh Harjani (IBM) <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Ralph Campbell <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: donettom-1

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

@@ -1657,7 +1657,7 @@ TEST_F(hmm2, double_map)
 
 	buffer->fd = -1;
 	buffer->size = size;
-	buffer->mirror = malloc(npages);
+	buffer->mirror = malloc(size);
 	ASSERT_NE(buffer->mirror, NULL);
 
 	/* Reserve a range of addresses. */