Messages in this thread | | | Date | Fri, 13 Jan 2012 17:55:27 -0500 (EST) | From | Nicolas Pitre <> | Subject | Re: [RFC PATCH] proc: clear_refs: do not clear reserved pages |
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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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