[PATCH] mm: avoid endless recursion in dump_page()

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

dump_page() uses page_mapcount() to get mapcount of the page.
page_mapcount() has VM_BUG_ON_PAGE(PageSlab(page)) as mapcount doesn't
make sense for slab pages and the field in struct page used for other
information.

It leads to recursion if dump_page() called for slub page and DEBUG_VM
is enabled:

dump_page() -> page_mapcount() -> VM_BUG_ON_PAGE() -> dump_page -> ...

Let's avoid calling page_mapcount() for slab pages in dump_page().

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 mm/debug.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/debug.c b/mm/debug.c
index 8865bfb41b0b..74c7cae4f683 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -42,9 +42,11 @@ const struct trace_print_flags vmaflag_names[] = {
 
 void __dump_page(struct page *page, const char *reason)
 {
+	int mapcount = PageSlab(page) ? 0 : page_mapcount(page);
+
 	pr_emerg("page:%p count:%d mapcount:%d mapping:%p index:%#lx",
-		  page, page_ref_count(page), page_mapcount(page),
-		  page->mapping, page->index);
+		  page, page_ref_count(page), mapcount,
+		  page->mapping, page_to_pgoff(page));
 	if (PageCompound(page))
 		pr_cont(" compound_mapcount: %d", compound_mapcount(page));
 	pr_cont("\n");
-- 
2.9.3

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