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

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Date:	Wed, 12 Aug 2009 18:59:42 +0530
From:	Nitin Gupta <ngupta@...are.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] swap: send callback when swap slot is freed

On 08/12/2009 06:26 PM, Ingo Molnar wrote:
> * Nitin Gupta<ngupta@...are.org>  wrote:
>
>> (Re-sending since I didn't get any reply last time)
>>
>> 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.

<snip>

>
> Shouldnt this be all be unconditional? Sounds like a reasonable VM
> callback. The patch would become a lot smaller as well.
>


I also think this can be made unconditional as it makes the patch
much cleaner. I made it conditional just to be on path of least
resistance :)

Thanks for looking into this. I will resend the patch removing
all the #ifdef'ery.

Nitin
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