linux-kernel - Re: [PATCH 2/7] ksm: treat unstable nid like in stable tree

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Date:	Fri, 22 Feb 2013 13:03:51 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Ric Mason <ric.masonn@...il.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	Petr Holasek <pholasek@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Izik Eidus <izik.eidus@...ellosystems.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 2/7] ksm: treat unstable nid like in stable tree

On Fri, 22 Feb 2013, Ric Mason wrote:
> On 02/21/2013 04:20 PM, Hugh Dickins wrote:
> > An inconsistency emerged in reviewing the NUMA node changes to KSM:
> > when meeting a page from the wrong NUMA node in a stable tree, we say
> > that it's okay for comparisons, but not as a leaf for merging; whereas
> > when meeting a page from the wrong NUMA node in an unstable tree, we
> > bail out immediately.
> 
> IIUC
> - ksm page from the wrong NUMA node will be add to current node's stable tree

That should never happen (and when I was checking with a WARN_ON it did
not happen).  What can happen is that a node already in a stable tree
has its page migrated away to another NUMA node.

> - normal page from the wrong NUMA node will be merged to current node's
> stable tree  <- where I miss here? I didn't see any special handling in
> function stable_tree_search for this case.

	nid = get_kpfn_nid(page_to_pfn(page));
	root = root_stable_tree + nid;

to choose the right tree for the page, and

				if (get_kpfn_nid(stable_node->kpfn) !=
						NUMA(stable_node->nid)) {
					put_page(tree_page);
					goto replace;
				}

to make sure that we don't latch on to a node whose page got migrated away.

> - normal page from the wrong NUMA node will compare but not as a leaf for
> merging after the patch

I don't understand you there, but hope my remarks above resolve it.

Hugh
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