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

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

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.


> > 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.


Thanks,
-Kame

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