linux-kernel - Re: [PATCH 2/4] send callback when swap slot is freed

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Date:	Thu, 24 Sep 2009 10:39:19 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Nitin Gupta <ngupta@...are.org>
Cc:	Greg KH <greg@...ah.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Ed Tomlinson <edt@....ca>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	linux-mm-cc <linux-mm-cc@...top.org>
Subject: Re: [PATCH 2/4] send callback when swap slot is freed

On Fri, 18 Sep 2009 04:13:30 +0530
Nitin Gupta <ngupta@...are.org> wrote:

> @@ -585,6 +617,8 @@ static int swap_entry_free(struct swap_info_struct *p,
>  			swap_list.next = p - swap_info;
>  		nr_swap_pages++;
>  		p->inuse_pages--;
> +		if (p->swap_free_notify_fn)
> +			p->swap_free_notify_fn(p->bdev, offset);
>  	}
>  	if (!swap_count(count))
>  		mem_cgroup_uncharge_swap(ent);

A nitpick but I feel I have to explain why mem_cgroup_ucharge_swap() is called
here. (difference with p->swap_free_notify_fn)

if (!swap_count(count))

means "It seems no users for this swap entry but we're not sure there are
       SwapCache for this entry or not."

In mem_cgroup_uncharge_swap(), swap_cgroup is checked and if there is
a record (which means the SwapCache is not mapped anywhere), swap usage
is uncharged.

This is for race window at freeing swap entry via
zap_pte_range() => free_swap_and_cache().
(swap entry is not freed if the page is locked.)

I'll add some explanation in next series of memcg-cleanup patches.

Thanks,
-Kame

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