Re: [RFC PATCH] proc: clear_refs: do not clear reserved pages

[Nicolas Pitre]

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@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Nicolas Pitre <nico@xxxxxxxxxxx>
> Reported-by: Moussa Ba <moussaba@xxxxxxxxxx>
> Signed-off-by: Will Deacon <will.deacon@xxxxxxx>

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

Acked-by: Nicolas Pitre <nico@xxxxxxxxxx>

> 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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