linux-kernel - Re: [PATCH 1/11] ksm: allow trees per NUMA node

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Date:	Sat, 26 Jan 2013 21:16:21 -0600
From:	Simon Jeons <simon.jeons@...il.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Petr Holasek <pholasek@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Izik Eidus <izik.eidus@...ellosystems.com>,
	Rik van Riel <riel@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Anton Arapov <anton@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 1/11] ksm: allow trees per NUMA node

On Sat, 2013-01-26 at 18:54 -0800, Hugh Dickins wrote:
> On Sat, 26 Jan 2013, Simon Jeons wrote:
> > On Fri, 2013-01-25 at 17:54 -0800, Hugh Dickins wrote:
> > > From: Petr Holasek <pholasek@...hat.com>
> > > @@ -1122,6 +1166,18 @@ struct rmap_item *unstable_tree_search_i
> > >  			return NULL;
> > >  		}
> > >  
> > > +		/*
> > > +		 * If tree_page has been migrated to another NUMA node, it
> > > +		 * will be flushed out and put into the right unstable tree
> > 
> > Then why not insert the new page to unstable tree during page migration
> > against current upstream? Because default behavior is merge across
> > nodes.
> 
> I don't understand the words "against current upstream" in your question.

I mean current upstream codes without numa awareness. :)

> 
> We cannot move a page (strictly, a node) from one tree to another during
> page migration itself, because the necessary ksm_thread_mutex is not held.
> Not would we even want to while "merge across nodes".
> 
> Ah, perhaps you are pointing out that in current upstream, the only user
> of ksm page migration is memory hotremove, which in current upstream does
> hold ksm_thread_mutex.
> 
> So you'd like us to add code for moving a node from one tree to another
> in ksm_migrate_page() (and what would it do when it collides with an

Without numa awareness, I still can't understand your explanation why
can't insert the node to the tree just after page migration instead of
inserting it at the next scan.

> existing node?), code which will then be removed a few patches later
> when ksm page migration is fully enabled?
> 
> No, I'm not going to put any more thought into that.  When Andrea pointed
> out the problem with Petr's original change to ksm_migrate_page(), I did
> indeed think that we could do something cleverer at that point; but once
> I got down to trying it, found that a dead end.  I wasn't going to be
> able to test the hotremove case properly anyway, so no good pursuing
> solutions that couldn't be generalized.
> 
> Hugh


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