linux-kernel - Re: [PATCH 2/3] Add swap slot free callback to block_device

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Date:	Tue, 18 May 2010 09:01:40 +0530
From:	Nitin Gupta <ngupta@...are.org>
To:	Minchan Kim <minchan.kim@...il.com>
CC:	Greg KH <greg@...ah.com>, Pekka Enberg <penberg@...helsinki.fi>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nigel Cunningham <nigel@...onice.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Cyp <cyp561@...il.com>, driverdev <devel@...verdev.osuosl.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] Add swap slot free callback to block_device_operations

On 05/17/2010 05:31 PM, Minchan Kim wrote:
> On Mon, May 17, 2010 at 2:32 PM, Nitin Gupta <ngupta@...are.org> wrote:
>> This callback is required when RAM based devices are used as swap disks.
>> One such device is ramzswap 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.
>>
>> Signed-off-by: Nitin Gupta <ngupta@...are.org>
> Reviewed-by: Minchan Kim <minchan.kim@...il.com>
> 
> Looks good to me about code. so I added my review sign.
> But I have some comments.
> 
> last time I said, I don't like there is a swap specific function in
> block_device_operations.
> It doesn't need many memory but it's not good about design sine
> block_device_operations have common functions about block device.
> 
> But I don't have any good idea now where I put swap specific function.
> And Linus already acked this idea. Hmm.
> 
> If there isn't any objection, I don't insist on my thought.
> 
> Nitpick :
> AFAIR, Nitin introduced SWP_BLKDEV since he think access of long
> pointers isn't good.  ex)
> S_ISBLK(swap_info_struct->swap_file->f_mapping->host->i_mode)
> 
> But now, we have to access  p->bdev->bd_disk->fops->swap_slot_free_notify.
> Isn't it all right?
> 

I'm also not sure about this point but accessing yet another very long
pointer chain just to check if its a block device seems weird.

Anyways, its trivial to remove this swap flag if its later decided that
its not really needed.

Thanks,
Nitin
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