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

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Message-ID: <alpine.LNX.2.00.1303052031320.29433@eggly.anvils>
Date:	Tue, 5 Mar 2013 21:05:10 -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 Wed, 6 Mar 2013, Ric Mason wrote:
[ I've deleted the context because that was about the unstable tree,
  and here you have moved to asking about a case in the stable tree. ]
> 
> For the case of a ksm page is migrated to a different NUMA node and migrate
> its stable node to  the right tree and collide with an existing stable node.
> get_kpfn_nid(stable_node->kpfn) != NUMA(stable_node->nid) can capture nothing

That's not so: as I've pointed out before, ksm_migrate_page() updates
stable_node->kpfn for the new page on the new NUMA node; but it cannot
(get the right locking to) move the stable_node to its new tree at that time.

It's moved out once ksmd notices that it's in the wrong NUMA node tree -
perhaps when one its rmap_items reaches the head of cmp_and_merge_page(),
or perhaps here in stable_tree_search() when it matches another page
coming in to cmp_and_merge_page().

You may be concentrating on the case when that "another page" is a ksm
page migrated from a different NUMA node; and overlooking the case of
when the matching ksm page in this stable tree has itself been migrated.

> since stable_node is the node in the right stable tree, nothing happen to it
> before this check. Did you intend to check get_kpfn_nid(page_node->kpfn) !=
> NUMA(page_node->nid) ?

Certainly not: page_node is usually NULL.  But I could have checked
get_kpfn_nid(stable_node->kpfn) != nid: I was duplicating the test
from cmp_and_merge_page(), but here we do have local variable nid.

Hugh
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