Messages in this thread Patch in this message | | | Date | Wed, 28 Aug 2013 18:41:00 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH -v3] sched, numa: Use {cpu, pid} to create task groups for shared faults |
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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.
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--- 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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