linux-kernel - Re: [PATCH 1/2] [mmotm] Add swap slot free callback to block_device

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Date:	Wed, 06 Jan 2010 19:10:02 +0530
From:	Nitin Gupta <ngupta@...are.org>
To:	Hugh Dickins <hugh.dickins@...cali.co.uk>
CC:	Pekka Enberg <penberg@...helsinki.fi>,
	Al Viro <viro@...IV.linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] [mmotm] Add swap slot free callback to block_device_operations


On 01/05/2010 04:28 PM, Hugh Dickins wrote:
> On Tue, 5 Jan 2010, Nitin Gupta wrote:
> 
>> This callback is required when RAM based devices are
>> used as swap disks. One such device is ramzswap[1] which
>> is used as compressed in-memory swap disk. For such
>> devices, we need a callback as soon as a swap slot is no
>> longer used to allow freeing memory allocated for this
>> slot. Without this callback, stale data can quickly
>> accumulate in memory defeating the whole purpose of
>> such devices.
>>
>> Another user of this callback will be "preswap" as
>> introduced by "Transcendent Memory" patches:
>> http://lwn.net/Articles/367286/
>> (I intend to integrade preswap with ramzswap).
>>
>> [1] ramzswap: http://code.google.com/p/compcache/
>>
>> Signed-off-by: Nitin Gupta <ngupta@...are.org>
> 
> Yes, thanks for your patience in putting this together:
> I much prefer this version to all the other ways considered.
> 

> (Though you should later add a comment, pointing out that this
> callback is with swap_lock and often page table lock also held.)
> 

I will send another patch for this.

> We know that's unsatisfactory, but it is the nature of swap:
> cutting some corners to get the job done without allocating
> more memory.  Having tried myself to do proper discard at
> swap_free time, I know that is hard: and I believe you've
> made the right decision to do it in this way for ramzswap.
> 
> Acked-by: Hugh Dickins <hugh.dickins@...cali.co.uk>
> 

Thanks for the review.
So, is this now going in mmotm tree?

Nitin
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