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

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Date:	Fri, 01 Mar 2013 13:29:15 +0800
From:	Ric Mason <ric.masonn@...il.com>
To:	Hugh Dickins <hughd@...gle.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


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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