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    SubjectRe: [RFC PATCH] proc: clear_refs: do not clear reserved pages
    On Fri, 13 Jan 2012, Will Deacon wrote:

    > /proc/pid/clear_refs is used to clear the Referenced and YOUNG bits for
    > pages and corresponding page table entries of the task with PID pid,
    > which includes any special mappings inserted into the page tables in
    > order to provide things like vDSOs and user helper functions.
    >
    > On ARM this causes a problem because the vectors page is mapped as a
    > global mapping and since ec706dab ("ARM: add a vma entry for the user
    > accessible vector page"), a VMA is also inserted into each task for this
    > page to aid unwinding through signals and syscall restarts. Since the
    > vectors page is required for handling faults, clearing the YOUNG bit
    > (and subsequently writing a faulting pte) means that we lose the vectors
    > page *globally* and cannot fault it back in. This results in a system
    > deadlock on the next exception.
    >
    > This patch avoids clearing the aforementioned bits for reserved pages,
    > therefore leaving the vectors page intact on ARM. Since reserved pages
    > are not candidates for swap, this change should not have any impact on
    > the usefulness of clear_refs.
    >
    > Cc: David Rientjes <rientjes@google.com>
    > Cc: Andrew Morton <akpm@linux-foundation.org>
    > Cc: Nicolas Pitre <nico@fluxnic.net>
    > Reported-by: Moussa Ba <moussaba@micron.com>
    > Signed-off-by: Will Deacon <will.deacon@arm.com>

    Given Andrew's answer, this should be fine wrt Russell's concern.

    Acked-by: Nicolas Pitre <nico@linaro.org>

    > An aside: if you want to see this problem in action, just run:
    >
    > $ echo 1 > /proc/self/clear_refs
    >
    > on an ARM platform (as any user) and watch your system hang. I think this
    > has been the case since 2.6.37, so I'll CC stable once people are happy
    > with the fix.
    >
    > fs/proc/task_mmu.c | 3 +++
    > 1 files changed, 3 insertions(+), 0 deletions(-)
    >
    > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
    > index e418c5a..7dcd2a2 100644
    > --- a/fs/proc/task_mmu.c
    > +++ b/fs/proc/task_mmu.c
    > @@ -518,6 +518,9 @@ static int clear_refs_pte_range(pmd_t *pmd, unsigned long addr,
    > if (!page)
    > continue;
    >
    > + if (PageReserved(page))
    > + continue;
    > +
    > /* Clear accessed and referenced bits. */
    > ptep_test_and_clear_young(vma, addr, pte);
    > ClearPageReferenced(page);
    > --
    > 1.7.4.1
    >


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