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

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Date:	Wed, 2 Jan 2013 21:10:48 -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 Tue, 1 Jan 2013, Simon Jeons wrote:
> 
> Hi Petr and Hugh,
> 
> One offline question, thanks for your clarify.

Perhaps not as offline as you intended :)

> 
> How to understand age = (unsigned char)(ksm_scan.seqnr -
> rmap_item->address);? It used for what?

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.

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
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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