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    SubjectRe: [PATCH] [8/16] POISON: Add various poison checks in mm/memory.c
    On Tue, Apr 07, 2009 at 09:31:45PM +0200, Andi Kleen wrote:
    > On Tue, Apr 07, 2009 at 09:03:30PM +0200, Johannes Weiner wrote:
    > > On Tue, Apr 07, 2009 at 05:10:05PM +0200, Andi Kleen wrote:
    > > >
    > > > Bail out early when poisoned pages are found in page fault handling.
    > > > Since they are poisoned they should not be mapped freshly
    > > > into processes.
    > > >
    > > > This is generally handled in the same way as OOM, just a different
    > > > error code is returned to the architecture code.
    > > >
    > > > Signed-off-by: Andi Kleen <ak@linux.intel.com>
    > > >
    > > > ---
    > > > mm/memory.c | 7 +++++++
    > > > 1 file changed, 7 insertions(+)
    > > >
    > > > Index: linux/mm/memory.c
    > > > ===================================================================
    > > > --- linux.orig/mm/memory.c 2009-04-07 16:39:39.000000000 +0200
    > > > +++ linux/mm/memory.c 2009-04-07 16:39:39.000000000 +0200
    > > > @@ -2560,6 +2560,10 @@
    > > > goto oom;
    > > > __SetPageUptodate(page);
    > > >
    > > > + /* Kludge for now until we take poisoned pages out of the free lists */
    > > > + if (unlikely(PagePoison(page)))
    > > > + return VM_FAULT_POISON;
    > > > +
    > >
    > > When memory_failure() hits a page still on the free list
    >
    > It won't free it then. Later on it will take it out of the free lists,
    > but that code is not written yet.
    >
    > > (!page_count()) then the get_page() in memory_failure() will trigger a
    > > VM_BUG. So either this check is unneeded or it should be
    >
    > So no bug
    > > get_page_unless_zero() in memory_failure()?
    >
    > That's not what this is handling. The issue is that sometimes
    > the process can still freeing it and we need to make sure it
    > never hits the free lists.

    I think we missed each other here. I wasn't talking about _why_ you
    take that reference -- that is clear. But I see these two
    possibilities:

    a) memory_failure() is called on a page on the free list, the
    get_page() will trigger a bug because the refcount is 0

    b) if that is not possible, the above check is not needed


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