Re: [PATCH] mm: Fix NULL ptr dereference in __count_immobile_pages

[Michal Hocko <mhocko@suse.cz>]

On Thu 12-01-12 17:35:36, KAMEZAWA Hiroyuki wrote:
> On Thu, 12 Jan 2012 09:27:22 +0100
> Michal Hocko <mhocko@suse.cz> wrote:
> 
> > On Thu 12-01-12 11:17:02, KAMEZAWA Hiroyuki wrote:
> > > On Wed, 11 Jan 2012 09:48:02 +0100
> > > Michal Hocko <mhocko@suse.cz> wrote:
> > > 
> > > > On Tue 10-01-12 13:31:08, David Rientjes wrote:
> > > > > On Tue, 10 Jan 2012, Michal Hocko wrote:
> > > > [...]
> > > > > >  mm/page_alloc.c |   11 +++++++++++
> > > > > >  1 files changed, 11 insertions(+), 0 deletions(-)
> > > > > > 
> > > > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > > > > index 2b8ba3a..485be89 100644
> > > > > > --- a/mm/page_alloc.c
> > > > > > +++ b/mm/page_alloc.c
> > > > > > @@ -5608,6 +5608,17 @@ __count_immobile_pages(struct zone *zone, struct page *page, int count)
> > > > > >  bool is_pageblock_removable_nolock(struct page *page)
> > > > > >  {
> > > > > >  	struct zone *zone = page_zone(page);
> > > > > > +	unsigned long pfn = page_to_pfn(page);
> > > > > > +
> > > 
> > > Hmm, I don't like to use page_zone() when we know the page may not be initialized.
> > > Shouldn't we add
> > > 
> > > 	if (!node_online(page_to_nid(page))
> > > 		return false;
> > > ?
> > 
> > How is this different? The node won't be initialized in page flags as
> > well.
> > 
> 
> page_zone(page) is
> ==
> static inline struct zone *page_zone(const struct page *page)
> {
>         return &NODE_DATA(page_to_nid(page))->node_zones[page_zonenum(page)];
> }
> ==
> 
> Then, if the page is unitialized, 
> 
>    &(NODE_DATA(0)->node_zones[0])
> 
> If NODE_DATA(0) is NULL, node_zones[0] is NULL just because zone array is placed
> on the top of struct pglist_data.
> 
> I never think someone may change the layout but...Hmm, just a nitpick.
> please do as you like.

Yes, fair point. See the follow up patch bellow.

> > > But...hmm. I think we should return 'true' here for removing a section with a hole
> > > finally....(Now, false will be safe.)
> > 
> > Those pages are reserved (for BIOS I guess) in this particular case so I
> > do not think it is safe to claim that the block is removable. Or am I
> > missing something?
> > 
> 
> We can't know it's reserved by BIOS or it's just a memory hole by the fact
> the page wasn't initialized.

OK, so then we should return false to mark to block non removable,
right?
--- 
>From 04f74e6f0ebf28f61650d63f8884e8855fb21b55 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@suse.cz>
Date: Thu, 12 Jan 2012 10:19:04 +0100
Subject: [PATCH] mm: __count_immobile_pages make sure the node is online

page_zone requires to have an online node otherwise we are accessing
NULL NODE_DATA. This is not an issue at the moment because node_zones
are located at the structure beginning but this might change in the
future so better be careful about that.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
 mm/page_alloc.c |   11 +++++++++--
 1 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 485be89..c6fb8ea 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5607,14 +5607,21 @@ __count_immobile_pages(struct zone *zone, struct page *page, int count)
 
 bool is_pageblock_removable_nolock(struct page *page)
 {
-	struct zone *zone = page_zone(page);
-	unsigned long pfn = page_to_pfn(page);
+	struct zone *zone;
+	unsigned long pfn;
 
 	/*
 	 * We have to be careful here because we are iterating over memory
 	 * sections which are not zone aware so we might end up outside of
 	 * the zone but still within the section.
+	 * We have to take care about the node as well. If the node is offline
+	 * its NODE_DATA will be NULL - see page_zone.
 	 */
+	if (!node_online(page_to_nid(page)))
+		return false;
+
+	zone = page_zone(page);
+	pfn = page_to_pfn(page);
 	if (!zone || zone->zone_start_pfn > pfn ||
 			zone->zone_start_pfn + zone->spanned_pages <= pfn)
 		return false;
-- 
1.7.7.3

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic