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

[Ric Mason]

Hi Hugh,
On 02/23/2013 05:03 AM, Hugh Dickins wrote:
> 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

Please forgive my late response.

> 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.

I think the ksm implementation for num awareness  is buggy.

For page migratyion stuff, new page is allocated from node *which page 
is migrated to*.
- when meeting a page from the wrong NUMA node in an unstable tree
    get_kpfn_nid(page_to_pfn(page)) *==* page_to_nid(tree_page)
    How can say it's okay for comparisons, but not as a leaf for merging?
- when meeting a page from the wrong NUMA node in an stable tree
   - meeting a normal page
   - meeting a page which is ksm page before migration
     get_kpfn_nid(stable_node->kpfn) != NUMA(stable_node->nid) can't 
capture them since stable_node is for tree page in current stable tree. 
They are always equal.
> > - 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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