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

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Date:	Mon, 17 Aug 2009 10:38:28 +0530
From:	Nitin Gupta <ngupta@...are.org>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
CC:	mingo@...e.hu, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] swap: send callback when swap slot is freed

On 08/17/2009 08:25 AM, KAMEZAWA Hiroyuki wrote:
> On Wed, 12 Aug 2009 20:07:43 +0530
> Nitin Gupta<ngupta@...are.org>  wrote:
>
>> Currently, we have "swap discard" mechanism which sends a discard bio request
>> when we find a free cluster during scan_swap_map(). This callback can come a
>> long time after swap slots are actually freed.
>>
>> This delay in callback is a great problem when (compressed) RAM [1] is used
>> as a swap device. So, this change adds a callback which is called as
>> soon as a swap slot becomes free. For above mentioned case of swapping
>> over compressed RAM device, this is very useful since we can immediately
>> free memory allocated for this swap page.
>>
>> This callback does not replace swap discard support. It is called with
>> swap_lock held, so it is meant to trigger action that finishes quickly.
>> However, swap discard is an I/O request and can be used for taking longer
>> actions.
>>
>> Links:
>> [1] http://code.google.com/p/compcache/
>>
>
> Hmm, do you really need notify at *every* swap free ?
> No batching is necessary ?
>

We need notify for every swap free and no batching is desired, at least for 
compcache case.

Thanks,
Nitin
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