[PATCH] mm/sparse: reset section's mem_map when fully deactivated

[Pingfan Liu <kernelfans@gmail.com>]

When fully deactivated, it is meaningless to keep the value of a section's
mem_map. And its mem_map will be reassigned during re-added.

Beside this, it breaks the user space tool "makedumpfile", which makes
assumption that a hot-removed section having mem_map as NULL.

The bug can be reproduced on IBM POWERVM by "drmgr -c mem -r -q 5" ,
trigger a crash, and save vmcore by makedumpfile

Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
To: linux-mm@kvack.org
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: kexec@lists.infradead.org
Cc: Kazuhito Hagio <k-hagio@ab.jp.nec.com>
---
 mm/sparse.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/sparse.c b/mm/sparse.c
index 3822ecb..fddac80 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -789,7 +789,7 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
 			ms->usage = NULL;
 		}
 		memmap = sparse_decode_mem_map(ms->section_mem_map, section_nr);
-		ms->section_mem_map = sparse_encode_mem_map(NULL, section_nr);
+		ms->section_mem_map = NULL;
 	}
 
 	if (section_is_early && memmap)
-- 
2.7.5