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

[Hugh Dickins <hughd@google.com>]

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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