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


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