linux-kernel - Re: [PATCH v7 1/2] KSM: numa awareness sysfs knob

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Date:	Fri, 4 Jan 2013 15:03:28 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Simon Jeons <simon.jeons@...il.com>
cc:	Petr Holasek <pholasek@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Izik Eidus <izik.eidus@...ellosystems.com>,
	Rik van Riel <riel@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Anton Arapov <anton@...hat.com>
Subject: Re: [PATCH v7 1/2] KSM: numa awareness sysfs knob

On Thu, 3 Jan 2013, Simon Jeons wrote:
> On Wed, 2013-01-02 at 21:10 -0800, Hugh Dickins wrote:
> > 
> > As you can see, remove_rmap_item_from_tree uses it to decide whether
> > or not it should rb_erase the rmap_item from the unstable_tree.
> > 
> > Every full scan of all the rmap_items, we increment ksm_scan.seqnr,
> > forget the old unstable_tree (it would just be a waste of processing
> > to remove every node one by one), and build up the unstable_tree afresh.
> > 
> 
> When the rmap_items left over from the previous scan will be removed?

Removed from the unstable rbtree?  Not at all, it's simply restarted
afresh, and the old rblinkages ignored.  Freed back to slab?  When the
scan passes that mm+address and realizes that rmap_item is not wanted
any more.  (Or when ksm is shut down with KSM_RUN_UNMERGE.)

> 
> > That works fine until we need to remove an rmap_item: then we have to be
> > very sure to remove it from the unstable_tree if it's already been linked
> > there during this scan, but ignore its rblinkage if that's just left over
> > from the previous scan.
> > 
> > A single bit would be enough to decide this; but we got it troublesomely
> > wrong in the early days of KSM (didn't always visit every rmap_item each
> > scan), so it's convenient to use 8 bits (the low unsigned char, stored
> 
> When the scenario didn't always visit every rmap_item each scan can
> occur? 

You're asking me about a stage of KSM development 3.5 years ago:
I don't remember the details.

> 
> > below the FLAGs and below the page-aligned address in the rmap_item -
> > there's lots of them, best keep them as small as we can) and do a
> > BUG_ON(age > 1) if we made a mistake.
> > 
> > We haven't hit that BUG_ON in over three years: if we need some more
> > bits for something, we can cut the age down to one or two bits.
> > 
> > Hugh
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