mm/hmm/mirror: device page fault handler

This handles page fault on behalf of device driver, unlike
handle_mm_fault() it does not trigger migration back to system memory for
device memory.

Link: http://lkml.kernel.org/r/20170817000548.32038-6-jglisse@redhat.comSigned-off-by: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Evgeny Baskakov <ebaskakov@nvidia.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Mark Hairgrove <mhairgrove@nvidia.com>
Signed-off-by: Sherry Cheung <SCheung@nvidia.com>
Signed-off-by: Subhash Gutti <sgutti@nvidia.com>
Cc: Aneesh Kumar <aneesh.kumar@linux.vnet.ibm.com>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: David Nellans <dnellans@nvidia.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Bob Liu <liubo95@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/hmm.h

@@ -292,6 +292,33 @@ int hmm_vma_get_pfns(struct vm_area_struct *vma,
 		     unsigned long end,
 		     hmm_pfn_t *pfns);
 bool hmm_vma_range_done(struct vm_area_struct *vma, struct hmm_range *range);
+
+
+/*
+ * Fault memory on behalf of device driver. Unlike handle_mm_fault(), this will
+ * not migrate any device memory back to system memory. The hmm_pfn_t array will
+ * be updated with the fault result and current snapshot of the CPU page table
+ * for the range.
+ *
+ * The mmap_sem must be taken in read mode before entering and it might be
+ * dropped by the function if the block argument is false. In that case, the
+ * function returns -EAGAIN.
+ *
+ * Return value does not reflect if the fault was successful for every single
+ * address or not. Therefore, the caller must to inspect the hmm_pfn_t array to
+ * determine fault status for each address.
+ *
+ * Trying to fault inside an invalid vma will result in -EINVAL.
+ *
+ * See the function description in mm/hmm.c for further documentation.
+ */
+int hmm_vma_fault(struct vm_area_struct *vma,
+		  struct hmm_range *range,
+		  unsigned long start,
+		  unsigned long end,
+		  hmm_pfn_t *pfns,
+		  bool write,
+		  bool block);
 #endif /* IS_ENABLED(CONFIG_HMM_MIRROR) */