[PATCH] mm: pass VM_BUG_ON() reason to dump_page()

[Dave Hansen]

From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

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
	...


Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
---

 b/include/linux/mmdebug.h |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff -puN include/linux/mmdebug.h~pass-VM_BUG_ON-reason-to-dump_page include/linux/mmdebug.h
--- a/include/linux/mmdebug.h~pass-VM_BUG_ON-reason-to-dump_page	2014-04-11 11:20:37.175240200 -0700
+++ b/include/linux/mmdebug.h	2014-04-11 13:11:09.498620420 -0700
@@ -9,8 +9,13 @@ extern void dump_page_badflags(struct pa
 
 #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)
 #else
 #define VM_BUG_ON(cond) BUILD_BUG_ON_INVALID(cond)
 #define VM_BUG_ON_PAGE(cond, page) VM_BUG_ON(cond)
_
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