Re: [PATCH] [v2] mm: pass VM_BUG_ON() reason to dump_page()

[Andrew Morton]

On Fri, 11 Apr 2014 13:42:32 -0700 Dave Hansen <dave@xxxxxxxx> wrote:

> 
> 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
> 	...
>
> ...
>
> --- a/include/linux/mmdebug.h~pass-VM_BUG_ON-reason-to-dump_page	2014-04-11 13:39:26.313125298 -0700
> +++ b/include/linux/mmdebug.h	2014-04-11 13:40:26.417835916 -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)

This seems prudent:

--- a/include/linux/mmdebug.h~mm-pass-vm_bug_on-reason-to-dump_page-fix
+++ a/include/linux/mmdebug.h
@@ -1,6 +1,8 @@
 #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);
_

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