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

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Date:	Fri, 1 Mar 2013 18:57:51 -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 Sat, 2 Mar 2013, Ric Mason wrote:
> On 03/02/2013 04:03 AM, Hugh Dickins wrote:
> > On Fri, 1 Mar 2013, Ric Mason wrote:
> > > I think the ksm implementation for num awareness  is buggy.
> > Sorry, I just don't understand your comments below,
> > but will try to answer or question them as best I can.
> > 
> > > For page migratyion stuff, new page is allocated from node *which page is
> > > migrated to*.
> > Yes, by definition.
> > 
> > > - 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)
> > I thought you were writing of the wrong NUMA node case,
> > but now you emphasize "*==*", which means the right NUMA node.
> 
> Yes, I mean the wrong NUMA node. During page migration, new page has already
> been allocated in new node and old page maybe freed.  So tree_page is the
> page in new node's unstable tree, page is also new node page, so
> get_kpfn_nid(page_to_pfn(page)) *==* page_to_nid(tree_page).

I don't understand; but here you seem to be describing a case where two
pages from the same NUMA node get merged (after both have been migrated
from another NUMA node?), and there's nothing wrong with that,
so I won't worry about it further.

> > >     - 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.
> > When we meet a ksm page in the stable tree before it's migrated to another
> > NUMA node, yes, it will be on the right NUMA node (because we were careful
> > only to merge pages from the right NUMA node there), and that test will not
> > capture them.  It's for capturng a ksm page in the stable tree after it has
> > been migrated to another NUMA node.
> 
> ksm page migrated to another NUMA node still not freed, why? Who take page
> count of it?

The old page, the one which used to be a ksm page on the old NUMA node,
should be freed very soon: since it was isolated from lru, and its page
count checked, I cannot think of anything to hold a reference to it,
apart from migration itself - so it just needs to reach putback_lru_page(),
and then may rest awhile on __lru_cache_add()'s pagevec before being freed.

But I don't see where I said the old page was still not freed.

> If not  freed, since new page is allocated in new node, it is
> the copy of current ksm page, so current ksm doesn't change,
> get_kpfn_nid(stable_node->kpfn) *==* NUMA(stable_node->nid).

But ksm_migrate_page() did
		VM_BUG_ON(stable_node->kpfn != page_to_pfn(oldpage));
		stable_node->kpfn = page_to_pfn(newpage);
without changing stable_node->nid.

Hugh
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