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

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Message-ID: <20071221010653.GA19691@waste.org>
Date:	Thu, 20 Dec 2007 19:06:55 -0600
From:	Matt Mackall <mpm@...enic.com>
To:	David Miller <davem@...emloft.net>
Cc:	m.kozlowski@...land.pl, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, sparclinux@...r.kernel.org
Subject: Re: 2.6.24-rc5-mm1: problems with cat /proc/kpageflags

On Thu, Dec 20, 2007 at 04:17:26PM -0800, David Miller wrote:
> From: Mariusz Kozlowski <m.kozlowski@...land.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.
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