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

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Message-Id: <20091130094616.8f3d94a7.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Mon, 30 Nov 2009 09:46:16 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Hugh Dickins <hugh.dickins@...cali.co.uk>
Cc:	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,
	"kosaki.motohiro@...fujitsu.com" <kosaki.motohiro@...fujitsu.com>
Subject: Re: [PATCH 2/9] ksm: let shared pages be swappable

On Tue, 24 Nov 2009 16:42:15 +0000 (GMT)
Hugh Dickins <hugh.dickins@...cali.co.uk> wrote:
> +int page_referenced_ksm(struct page *page, struct mem_cgroup *memcg,
> +			unsigned long *vm_flags)
> +{
> +	struct stable_node *stable_node;
> +	struct rmap_item *rmap_item;
> +	struct hlist_node *hlist;
> +	unsigned int mapcount = page_mapcount(page);
> +	int referenced = 0;
> +	struct vm_area_struct *vma;
> +
> +	VM_BUG_ON(!PageKsm(page));
> +	VM_BUG_ON(!PageLocked(page));
> +
> +	stable_node = page_stable_node(page);
> +	if (!stable_node)
> +		return 0;
> +

Hmm. I'm not sure how many pages are shared in a system but
can't we add some threshold for avoidng too much scan against shared pages ?
(in vmscan.c)
like..
      
       if (page_mapcount(page) > (XXXX >> scan_priority))
		return 1;

I saw terrible slow downs in shmem-swap-out in old RHELs (at user support).
(Added kosaki to CC.)

After this patch, the number of shared swappable page will be unlimited.

Thanks,
-Kame


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