Re: 2.6.24-rc5-mm1: problems with cat /proc/kpageflags

Matt Mackall
2007-12-21T01:07:30Z

On Thu, Dec 20, 2007 at 04:17:26PM -0800, David Miller wrote:
> From: Mariusz Kozlowski <m.kozlowski@tuxland.pl>
> Date: Thu, 20 Dec 2007 20:47:55 +0100
>
> > [ 145.128915] TSTATE: 0000004411009603 TPC: 00000000005119ac TNPC: 00000000005119b0 Y: 00000000 Not tainted
> > [ 145.128940] TPC: <kpagecount_read+0x94/0xe0>
>
> My suspicion at this point is that with certain RAM layouts, simply
> iterating over PFN's is simply not working out.

That was my original suspicion, which is why I asked Mariusz to
effectively comment out the actual PFN lookup up-thread. I didn't send
him a patch to do that, so I guess my instructions on how to hack it
may have been misunderstood.

> pfn_to_page() seems to be doing no range checking, and with sparsemem
> vmemmap, which sparc64 always uses, this can be problematic.
>
> It just blindly goes "vmemmap + pfn" which is asking for trouble, in
> particular when the physical RAM layout really is sparse.
>
> Maybe it's enough to add a pfn_valid() check here? If pfn_valid()
> means there is a vmemmap translation setup for that page struct too,
> it would work.

Here's a test patch:

Index: mm/fs/proc/proc_misc.c
===================================================================
--- mm.orig/fs/proc/proc_misc.c 2007-12-20 19:04:35.000000000 -0600
+++ mm/fs/proc/proc_misc.c 2007-12-20 19:06:01.000000000 -0600
@@ -707,7 +707,10 @@ static ssize_t kpagecount_read(struct fi
return -EIO;

while (count > 0) {
- ppage = pfn_to_page(pfn++);
+ ppage = 0;
+ if (pfn_valid(pfn))
+ ppage = pfn_to_page(pfn);
+ pfn++;
if (!ppage)
pcount = 0;
else
@@ -773,7 +776,10 @@ static ssize_t kpageflags_read(struct fi
return -EIO;

while (count > 0) {
- ppage = pfn_to_page(pfn++);
+ ppage = 0;
+ if (pfn_valid(pfn))
+ ppage = pfn_to_page(pfn);
+ pfn++;
if (!ppage)
kflags = 0;
else

--
Mathematics is the supreme nostalgia of our time.