mm/hmm: add a test for cross device private faults

Add a simple test case for when hmm_range_fault() is called with the
HMM_PFN_REQ_FAULT flag and a device private PTE is found for a device
other than the hmm_range::dev_private_owner.  This should cause the page
to be faulted back to system memory from the other device and the PFN
returned in the output array.

Also, remove a piece of code that unnecessarily unmaps part of the buffer.

Link: https://lkml.kernel.org/r/20220727000837.4128709-3-rcampbell@nvidia.com
Link: https://lkml.kernel.org/r/20220725183615.4118795-3-rcampbell@nvidia.com
Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: Alistair Popple <apopple@nvidia.com>
Cc: Felix Kuehling <felix.kuehling@amd.com>
Cc: Philip Yang <Philip.Yang@amd.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Ralph Campbell

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

@@ -1603,9 +1603,19 @@ TEST_F(hmm2, double_map)
 	for (i = 0, ptr = buffer->mirror; i < size / sizeof(*ptr); ++i)
 		ASSERT_EQ(ptr[i], i);
 
-	/* Punch a hole after the first page address. */
-	ret = munmap(buffer->ptr + self->page_size, self->page_size);
+	/* Migrate pages to device 1 and try to read from device 0. */
+	ret = hmm_dmirror_cmd(self->fd1, HMM_DMIRROR_MIGRATE, buffer, npages);
+	ASSERT_EQ(ret, 0);
+	ASSERT_EQ(buffer->cpages, npages);
+
+	ret = hmm_dmirror_cmd(self->fd0, HMM_DMIRROR_READ, buffer, npages);
 	ASSERT_EQ(ret, 0);
+	ASSERT_EQ(buffer->cpages, npages);
+	ASSERT_EQ(buffer->faults, 1);
+
+	/* Check what device 0 read. */
+	for (i = 0, ptr = buffer->mirror; i < size / sizeof(*ptr); ++i)
+		ASSERT_EQ(ptr[i], i);
 
 	hmm_buffer_free(buffer);
 }