mm/migrate: allow migrate_vma() to alloc new page on empty entry

This allows callers of migrate_vma() to allocate new page for empty CPU
page table entry (pte_none or back by zero page).  This is only for
anonymous memory and it won't allow new page to be instanced if the
userfaultfd is armed.

This is useful to device driver that want to migrate a range of virtual
address and would rather allocate new memory than having to fault later
on.

Link: http://lkml.kernel.org/r/20170817000548.32038-18-jglisse@redhat.com
Signed-off-by: Jérôme Glisse <jglisse@redhat.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: Evgeny Baskakov <ebaskakov@nvidia.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Mark Hairgrove <mhairgrove@nvidia.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: Sherry Cheung <SCheung@nvidia.com>
Cc: Subhash Gutti <sgutti@nvidia.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>

1 files changed, 9 insertions, 0 deletions

diff --git a/include/linux/migrate.h b/include/linux/migrate.h
index 8dc8f0a3f1af..d4e6d12a0b40 100644
--- a/include/linux/migrate.h
+++ b/include/linux/migrate.h
@@ -218,6 +218,15 @@ static inline unsigned long migrate_pfn(unsigned long pfn)
  * driver should avoid setting MIGRATE_PFN_ERROR unless it is really in an
  * unrecoverable state.
  *
+ * For empty entry inside CPU page table (pte_none() or pmd_none() is true) we
+ * do set MIGRATE_PFN_MIGRATE flag inside the corresponding source array thus
+ * allowing device driver to allocate device memory for those unback virtual
+ * address. For this the device driver simply have to allocate device memory
+ * and properly set the destination entry like for regular migration. Note that
+ * this can still fails and thus inside the device driver must check if the
+ * migration was successful for those entry inside the finalize_and_map()
+ * callback just like for regular migration.
+ *
  * THE alloc_and_copy() CALLBACK MUST NOT CHANGE ANY OF THE SRC ARRAY ENTRIES
  * OR BAD THINGS WILL HAPPEN !
  *