linux-kernel - Re: [PATCH 2/6] ksm: dont allow overlap memory addresses registrations.

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Date:	Wed, 6 May 2009 15:46:31 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Izik Eidus <ieidus@...hat.com>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, chrisw@...hat.com, device@...ana.org,
	linux-mm@...ck.org, nickpiggin@...oo.com.au
Subject: Re: [PATCH 2/6] ksm: dont allow overlap memory addresses registrations.

On Wed, 6 May 2009, Alan Cox wrote:
> > the max number of ksm pages that can be allocated at any given time so
> > to avoid OOM conditions, like the swap-compress logic that limits the
> > swapdevice size to less than ram.

(I don't know anything about that swap-compress logic and limitation.)

> 
> Are those pages accounted for in the vm_overcommit logic, as if you
> allocate a big chunk of memory as KSM will do you need the worst case
> vm_overcommit behaviour preserved and that means keeping the stats
> correct.

As I understand it, KSM won't affect the vm_overcommit behaviour at all.
Those pages Izik refers to are not allocated up front, they're just a
limit on the number of process pages which may get held in core at any
one time, through being shared via the KSM mechanism.

KSM is not evading vm_committed_space at all, not opening a backdoor
away from the ordinary mmaps: just collapsing duplicated pages in
what's been mapped in the usual way, down to single copies.

So the vm_commited_space accounting is exactly as before: it would
be a bit odd to be running KSM along with OVERCOMMIT_NEVER, but it
doesn't change its calculations at all - it will and will have to
be as pessimistic as it ever was.

The only difference would be in how much memory (mostly lowmem)
KSM's own data structures will take up - as usual, the kernel
data structures aren't being accounted, but do take up memory.

Hugh
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