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