[PATCH] mm: Fix __dump_page() for poisoned pages

[Robin Murphy <robin.murphy@arm.com>]

Evaluating page_mapping() on a poisoned page ends up dereferencing junk
and making PF_POISONED_CHECK() considerably crashier than intended. Fix
that by not inspecting the mapping until we've determined that it's
likely to be valid.

Fixes: 1c6fb1d89e73 ("mm: print more information about mapping in __dump_page")
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
---
 mm/debug.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/debug.c b/mm/debug.c
index 0abb987dad9b..1611cf00a137 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -44,7 +44,7 @@ const struct trace_print_flags vmaflag_names[] = {
 
 void __dump_page(struct page *page, const char *reason)
 {
-	struct address_space *mapping = page_mapping(page);
+	struct address_space *mapping;
 	bool page_poisoned = PagePoisoned(page);
 	int mapcount;
 
@@ -58,6 +58,8 @@ void __dump_page(struct page *page, const char *reason)
 		goto hex_only;
 	}
 
+	mapping = page_mapping(page);
+
 	/*
 	 * Avoid VM_BUG_ON() in page_mapcount().
 	 * page->_mapcount space in struct page is used by sl[aou]b pages to
-- 
2.20.1.dirty