linux-kernel - Re: [PATCH 2/9] ksm: let shared pages be swappable

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Date:	Mon, 30 Nov 2009 12:38:33 +0000 (GMT)
From:	Hugh Dickins <hugh.dickins@...cali.co.uk>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Izik Eidus <ieidus@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Chris Wright <chrisw@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 2/9] ksm: let shared pages be swappable

On Mon, 30 Nov 2009, KOSAKI Motohiro wrote:
> > After this patch, the number of shared swappable page will be unlimited.
> 
> Probably, it doesn't matter. I mean
> 
>   - KSM sharing and Shmem sharing are almost same performance characteristics.
>   - if memroy pressure is low, SplitLRU VM doesn't scan anon list so much.
> 
> if ksm swap is too costly, we need to improve anon list scanning generically.

Yes, we're in agreement that this issue is not new with KSM swapping.

> btw, I'm not sure why bellow kmem_cache_zalloc() is necessary. Why can't we
> use stack?

Well, I didn't use stack: partly because I'm so ashamed of the pseudo-vmas
on the stack in mm/shmem.c, which have put shmem_getpage() into reports
of high stack users (I've unfinished patches to deal with that); and
partly because page_referenced_ksm() and try_to_unmap_ksm() are on
the page reclaim path, maybe way down deep on a very deep stack.

But it's not something you or I should be worrying about: as the comment
says, this is just a temporary hack, to present a patch which gets KSM
swapping working in an understandable way, while leaving some corrections
and refinements to subsequent patches.  This pseudo-vma is removed in the
very next patch.

Hugh

> 
> ----------------------------
> +	/*
> +	 * Temporary hack: really we need anon_vma in rmap_item, to
> +	 * provide the correct vma, and to find recently forked instances.
> +	 * Use zalloc to avoid weirdness if any other fields are involved.
> +	 */
> +	vma = kmem_cache_zalloc(vm_area_cachep, GFP_ATOMIC);
> +	if (!vma) {
> +		spin_lock(&ksm_fallback_vma_lock);
> +		vma = &ksm_fallback_vma;
> +	}
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