libnvdimm/region: Enable MAP_SYNC for volatile regions

fix #27138800

commit 4c806b897d6075bfa5067e524fb058c57ab64e7b upstream.

Some environments want to use a host tmpfs/ramdisk to back guest pmem.
While the data is not persisted relative to the host it *is* persisted
relative to guest crashes / reboots. The guest is free to use dax and
MAP_SYNC to keep filesystem metadata consistent with dax accesses
without requiring guest fsync(). The guest can also observe that the
region is volatile and skip cache flushing as global visibility is
enough to "persist" data relative to the host staying alive over guest
reset events.
Signed-off-by: NAneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Reviewed-by: NPankaj Gupta <pagupta@redhat.com>
Link: https://lore.kernel.org/r/20190924114327.14700-1-aneesh.kumar@linux.ibm.com
[djbw: reword the changelog]
Signed-off-by: NDan Williams <dan.j.williams@intel.com>
Reviewed-by: NYang Shi <yang.shi@linux.alibaba.com>
Signed-off-by: NXunlei Pang <xlpang@linux.alibaba.com>

drivers/nvdimm/region_devs.c

@@ -1213,6 +1213,9 @@ EXPORT_SYMBOL_GPL(nvdimm_has_cache);
 
 bool is_nvdimm_sync(struct nd_region *nd_region)
 {
+	if (is_nd_volatile(&nd_region->dev))
+		return true;
+
 	return is_nd_pmem(&nd_region->dev) &&
 		!test_bit(ND_REGION_ASYNC, &nd_region->flags);
 }