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    Subject[PATCH] [5/5] Avoid theoretical vmalloc fault loop
    Date

    Ajith Kumar noticed:

    I was going through the vmalloc fault handling for x86_64 and am unclear
    about the following lines in the vmalloc_fault() function.

    pgd = pgd_offset(current->mm ?: &init_mm, address);
    pgd_ref = pgd_offset_k(address);

    Here the intention is to get the pgd corresponding to the current
    process and sync it up with the pgd in init_mm(obtained from
    pgd_offset_k). However, for kernel threads current->mm is NULL and hence
    pgd = pgd_offset(init_mm, address) = pgd_ref which means the fault
    handler returns without setting the pgd entry in the MM structure in
    the context of which the kernel thread has faulted. This could lead to
    never-ending faults and busy looping of kernel threads like pdflush.
    So, shouldn't the pgd = pgd_offset(current->mm ?: &init_mm, address);
    be
    pgd = pgd_offset(current->active_mm ?: &init_mm, address);

    AK: We can use active_mm unconditionally because it should
    be always set. Do that.

    Signed-off-by: Andi Kleen <ak@linux.intel.com>

    ---
    arch/x86/mm/fault.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    Index: linux-2.6.28-test/arch/x86/mm/fault.c
    ===================================================================
    --- linux-2.6.28-test.orig/arch/x86/mm/fault.c 2008-10-24 13:34:41.000000000 +0200
    +++ linux-2.6.28-test/arch/x86/mm/fault.c 2009-01-02 16:01:11.000000000 +0100
    @@ -533,7 +533,7 @@
    happen within a race in page table update. In the later
    case just flush. */

    - pgd = pgd_offset(current->mm ?: &init_mm, address);
    + pgd = pgd_offset(current->active_mm, address);
    pgd_ref = pgd_offset_k(address);
    if (pgd_none(*pgd_ref))
    return -1;

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