mm: pass VM_BUG_ON() reason to dump_page()

I recently added a patch to let folks pass a "reason" string dump_page()
which gets dumped out along with the page's data. This essentially
saves the bug-reader a trip in to the source to figure out why we
BUG_ON()'d.

The new VM_BUG_ON_PAGE() passes in NULL for "reason". It seems like we
might as well pass the BUG_ON() condition if we have it. This will
bloat kernels a bit with ~160 new strings, but this is all under a
debugging option anyway.

page:ffffea0008560280 count:1 mapcount:0 mapping:(null) index:0x0
page flags: 0xbfffc0000000001(locked)
page dumped because: VM_BUG_ON_PAGE(PageLocked(page))
------------[ cut here ]------------
kernel BUG at /home/davehans/linux.git/mm/filemap.c:464!
invalid opcode: 0000 [#1] SMP
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 3.14.0+ #251
Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
...

[[e-mail]: include stringify.h]
Signed-off-by: Dave Hansen <[e-mail]>
Acked-by: Kirill A. Shutemov <[e-mail]>
Acked-by: Davidlohr Bueso <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/mmdebug.h

···
 #ifndef LINUX_MM_DEBUG_H
 #define LINUX_MM_DEBUG_H 1
 
+#include <linux/stringify.h>
+
 struct page;
 
 extern void dump_page(struct page *page, const char *reason);
···
 
 #ifdef CONFIG_DEBUG_VM
 #define VM_BUG_ON(cond) BUG_ON(cond)
-#define VM_BUG_ON_PAGE(cond, page) \
-	do { if (unlikely(cond)) { dump_page(page, NULL); BUG(); } } while (0)
+#define VM_BUG_ON_PAGE(cond, page)					\
+	do {								\
+		if (unlikely(cond)) {					\
+			dump_page(page, "VM_BUG_ON_PAGE(" __stringify(cond)")");\
+			BUG();						\
+		}							\
+	} while (0)
 #define VM_WARN_ON(cond) WARN_ON(cond)
 #define VM_WARN_ON_ONCE(cond) WARN_ON_ONCE(cond)
 #else