powerpc/mm: Flush cache on memory hot(un)plug

This patch adds support for flushing potentially dirty cache lines
when memory is hot-plugged/hot-un-plugged. The support is currently
limited to 64 bit systems.

The bug was exposed when mappings for a device were actually
hot-unplugged and plugged in back later. A similar issue was observed
during the development of memtrace, but memtrace does it's own
flushing of region via a custom routine.

These patches do a flush both on hotplug/unplug to clear any stale
data in the cache w.r.t mappings, there is a small race window where a
clean cache line may be created again just prior to tearing down the
mapping.

The patches were tested by disabling the flush routines in memtrace
and doing I/O on the trace file. The system immediately
checkstops (quite reliablly if prior to the hot-unplug of the memtrace
region, we memset the regions we are about to hot unplug). After these
patches no custom flushing is needed in the memtrace code.

Fixes: 9d5171a ("powerpc/powernv: Enable removal of memory for in memory tracing")
Cc: stable@vger.kernel.org # v4.14+
Signed-off-by: Balbir Singh <bsingharora@gmail.com>
Acked-by: Reza Arbab <arbab@linux.ibm.com>
Reviewed-by: Rashmica Gupta <rashmica.g@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/mem.c

@@ -133,6 +133,7 @@ int __meminit arch_add_memory(int nid, u64 start, u64 size, struct vmem_altmap *
 			start, start + size, rc);
 		return -EFAULT;
 	}
+	flush_inval_dcache_range(start, start + size);
 
 	return __add_pages(nid, start_pfn, nr_pages, altmap, want_memblock);
 }
@@ -159,6 +160,7 @@ int __meminit arch_remove_memory(u64 start, u64 size, struct vmem_altmap *altmap
 
 	/* Remove htab bolted mappings for this section of memory */
 	start = (unsigned long)__va(start);
+	flush_inval_dcache_range(start, start + size);
 	ret = remove_section_mapping(start, start + size);
 
 	/* Ensure all vmalloc mappings are flushed in case they also