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

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Date:	Wed, 28 Aug 2013 18:41:00 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Don Morris <don.morris@...com>
Cc:	Mel Gorman <mgorman@...e.de>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>, riel@...hat.com
Subject: Re: [PATCH -v3] sched, numa: Use {cpu, pid} to create task groups
 for shared faults

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@...radead.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@...radead.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.

---

--- 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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