linux-kernel - Re: [RFC PATCH 02/02] swapon: add "cluster-discard" support

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Date:	Mon, 20 May 2013 21:02:43 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Rafael Aquini <aquini@...hat.com>
CC:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	akpm@...ux-foundation.org, hughd@...gle.com, shli@...nel.org,
	kzak@...hat.com, jmoyer@...hat.com, riel@...hat.com,
	lwoodman@...hat.com, mgorman@...e.de, kosaki.motohiro@...il.com
Subject: Re: [RFC PATCH 02/02] swapon: add "cluster-discard" support

> +.B "\-c, \-\-cluster\-discard"
> +Swapping will discard clusters of swap pages in between freeing them
> +and re-writing to them, if the swap device supports that. This option
> +also implies the
> +.I \-d, \-\-discard
> +swapon flag.

I'm not sure this is good idea. Why can't we make these flags orthogonal?


>  /* If true, don't complain if the device/file doesn't exist */
>  static int ifexists;
> @@ -570,8 +574,11 @@ static int do_swapon(const char *orig_special, int prio,
>  			   << SWAP_FLAG_PRIO_SHIFT);
>  	}
>  #endif
> -	if (fl_discard)
> +	if (fl_discard) {
>  		flags |= SWAP_FLAG_DISCARD;
> +		if (fl_discard > 1)
> +			flags |= SWAP_FLAG_DISCARD_CLUSTER;

This is not enough, IMHO. When running this code on old kernel, swapon() return EINVAL.
At that time, we should fall back swapon(0x10000).

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