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

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Date:	Wed, 06 Mar 2013 14:58:36 +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 03/06/2013 01:05 PM, Hugh Dickins wrote:
> 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

Are you sure? list_del(&page_node->list) and DO_NUMA(page_node->nid = 
nid) will trigger panic now.
> 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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