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[PATCH] mm/page_alloc: fix boot hang in memmap_init_zone

On some architectures (reported on arm64) commit 864b75f9d6b01 ("mm/page_alloc: fix memmap_init_zone pageblock alignment")
causes a boot hang. This patch fixes the hang making sure the alignment
never steps back.

Link: http://lkml.kernel.org/r/0485727b2e82da7efbce5f6ba42524b429d0391a.1520011945.git.neelx@xxxxxxxxxx
Fixes: 864b75f9d6b01 ("mm/page_alloc: fix memmap_init_zone pageblock alignment")
Signed-off-by: Daniel Vacek <neelx@xxxxxxxxxx>
Tested-by: Sudeep Holla <sudeep.holla@xxxxxxx>
Tested-by: Naresh Kamboju <naresh.kamboju@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Paul Burton <paul.burton@xxxxxxxxxx>
Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
 mm/page_alloc.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3d974cb2a1a1..e033a6895c6f 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5364,9 +5364,14 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
 			 * is not. move_freepages_block() can shift ahead of
 			 * the valid region but still depends on correct page
 			 * metadata.
+			 * Also make sure we never step back.
-			pfn = (memblock_next_valid_pfn(pfn, end_pfn) &
+			unsigned long next_pfn;
+			next_pfn = (memblock_next_valid_pfn(pfn, end_pfn) &
 					~(pageblock_nr_pages-1)) - 1;
+			if (next_pfn > pfn)
+				pfn = next_pfn;