Re: [PATCH -v3] sched, numa: Use {cpu, pid} to create task groups for shared faults

[Rik van Riel <riel@redhat.com>]

On 08/28/2013 12:41 PM, Peter Zijlstra wrote:
> On Fri, Aug 02, 2013 at 06:47:15PM +0200, Peter Zijlstra wrote:
>> Subject: sched, numa: Use {cpu, pid} to create task groups for shared faults
>> From: Peter Zijlstra <peterz@infradead.org>
>> Date: Tue Jul 30 10:40:20 CEST 2013
>>
>> A very simple/straight forward shared fault task grouping
>> implementation.
>>
>> Signed-off-by: Peter Zijlstra <peterz@infradead.org>
> 
> So Rik and me found a possible issue with this -- although in the end it
> turned out to be a userspace 'feature' instead.
> 
> It might be possible for a COW page to be 'shared' and thus get a
> last_cpupid set from another process. When we break cow and reuse the
> now private and writable page might still have this last_cpupid and thus
> cause a shared fault and form grouping.
> 
> Something like the below resets the last_cpupid field on reuse much like
> fresh COW copies will have.
> 
> There might be something that avoids the above scenario but I'm too
> tired to come up with anything.

I believe this is a real bug.

It can be avoided by either -1ing out the cpupid like you do, or
using the current process's cpupid, when we re-use an old page
in do_wp_page.

Acked-by: Rik van Riel <riel@redhat.com>

> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2730,6 +2730,9 @@ static int do_wp_page(struct mm_struct *
>  		get_page(dirty_page);
>  
>  reuse:
> +		if (old_page)
> +			page_cpupid_xchg_last(old_page, (1 << LAST_CPUPID_SHIFT) - 1);
> +
>  		flush_cache_page(vma, address, pte_pfn(orig_pte));
>  		entry = pte_mkyoung(orig_pte);
>  		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> 


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