linux-kernel - Re: [PATCH 01/02] swap: discard while swapping only if SWAP_FLAG_DISCARD

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Message-ID: <CAHGf_=oVFd3UZqL14tQoUoG+jfmV-NuCrhMv3VzCU1Sy0h9Y=w@mail.gmail.com>
Date:	Sun, 26 May 2013 12:02:45 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Rafael Aquini <aquini@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>, shli@...nel.org,
	Karel Zak <kzak@...hat.com>, Jeff Moyer <jmoyer@...hat.com>,
	"riel@...hat.com" <riel@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>,
	Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH 01/02] swap: discard while swapping only if SWAP_FLAG_DISCARD_PAGES

On Sun, May 26, 2013 at 11:30 AM, Rafael Aquini <aquini@...hat.com> wrote:
> On Sun, May 26, 2013 at 10:55:32AM -0400, KOSAKI Motohiro wrote:
>> On Sun, May 26, 2013 at 9:52 AM, Rafael Aquini <aquini@...hat.com> wrote:
>> > On Sun, May 26, 2013 at 07:44:56AM -0400, KOSAKI Motohiro wrote:
>> >> > +                       /*
>> >> > +                        * By flagging sys_swapon, a sysadmin can tell us to
>> >> > +                        * either do sinle-time area discards only, or to just
>> >> > +                        * perform discards for released swap page-clusters.
>> >> > +                        * Now it's time to adjust the p->flags accordingly.
>> >> > +                        */
>> >> > +                       if (swap_flags & SWAP_FLAG_DISCARD_ONCE)
>> >> > +                               p->flags &= ~SWP_PAGE_DISCARD;
>> >> > +                       else if (swap_flags & SWAP_FLAG_DISCARD_PAGES)
>> >> > +                               p->flags &= ~SWP_AREA_DISCARD;
>> >>
>> >> When using old swapon(8), this code turn off both flags, right
>> >
>>  > As the flag that enables swap discards SWAP_FLAG_DISCARD remains meaning the
>> > same it meant before, when using old swapon(8) (SWP_PAGE_DISCARD|SWP_AREA_DISCARD)
>>
>> But old swapon(8) don't use neigher SWAP_FLAG_DISCARD_ONCE nor
>> SWAP_FLAG_DISCARD_PAGES.  It uses only SWAP_FLAG_DISCARD. So, this
>> condition disables both SWP_PAGE_DISCARD and SWP_AREA_DISCARD.
>>
>
> This condition _only_ disables one of the new flags orthogonally if swapon(8)
> flags a policy to sys_swapon. As old swapon(8) can only flag SWAP_FLAG_DISCARD,
> the original behavior is kept. Nothing will change when one is using an old
> swapon(8) with this changeset.

Aha, got it. I misunderstood your code.
Thank you.

Acked-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
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