Re: show_mem panics in 2.4.22

[John Marvin <jsm@udlkern.fc.hp.com>]

> I'm running linux-2.4.22-ia64-030909 on an rx2600.  The show_mem()
> function always causes a kernel panic.  This is reached when you send
> 'SysRq m' or serial 'BREAK m' to find out about used memory, etc.
>
> The problem seems to be that this function is written assuming that
> the discontiguous memory scheme is used, but that's not the case in my
> configuration.  I see that in 2.6.0-test8 there are two versions of
> the function for the contig/discontig cases.  The crash is on the line
> that reads through pgdat->node_mem_map.  I'm not sure exactly what is
> wrong with that.


I'm not sure why this just started to show up. The problem is that
the size of struct page doesn't divide into the page size evenly, so
the structure overlaps holes in the mem_map array. Here is a fix,
but I am still not sure of the performance implications (extra memory
dereference). There may be a better fix, although not as simple, if
this has performance implications.

The same bug probably exists in 2.6.

John Marvin
jsm@fc.hp.com


--- a/arch/ia64/mm/init.c	Tue Oct 28 01:25:54 2003
+++ b/arch/ia64/mm/init.c	Tue Oct 28 01:31:26 2003
@@ -485,7 +485,8 @@ ia64_page_valid (struct page *page)
 {
 	char byte;
 
-	return __get_user(byte, (char *) page) = 0;
+	return     (__get_user(byte, (char *) page) = 0)
+		&& (__get_user(byte, (char *) (page + 1) - 1) = 0);
 }
 
 #define GRANULEROUNDDOWN(n) ((n) & ~(IA64_GRANULE_SIZE-1))