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    SubjectRe: [BUG 3.0.0-rc1] ksm: NULL pointer dereference in ksm_do_scan()
    * Andrea Righi (andrea@betterlinux.com) wrote:
    > mmh.. I can reproduce the bug also with the standard ubuntu (11.04)
    > kernel. Could you post your .config?

    Andrea (Righi), can you tell me if this WARN fires? This looks
    like a pure race between removing from list and checking list, i.e.
    insufficient locking.

    ksm_scan.mm_slot == the only registered mm

    CPU 1 (bug program) CPU 2 (ksmd)
    list_empty() is false
    lock
    ksm_scan.mm_slot
    list_del
    unlock
    slot == &ksm_mm_head (but list is now empty_)


    diff --git a/mm/ksm.c b/mm/ksm.c
    index 942dfc7..ab79a92 100644
    --- a/mm/ksm.c
    +++ b/mm/ksm.c
    @@ -1301,6 +1301,7 @@ static struct rmap_item *scan_get_next_rmap_item(struct page **page)
    slot = list_entry(slot->mm_list.next, struct mm_slot, mm_list);
    ksm_scan.mm_slot = slot;
    spin_unlock(&ksm_mmlist_lock);
    + WARN_ON(slot == &ksm_mm_head);
    next_mm:
    ksm_scan.address = 0;
    ksm_scan.rmap_list = &slot->rmap_list;

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