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

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Date:	Tue, 21 May 2013 18:06:32 -0300
From:	Rafael Aquini <aquini@...hat.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.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
Subject: Re: [RFC PATCH 01/02] swap: discard while swapping only if
 SWAP_FLAG_DISCARD_CLUSTER

Howdy Kosaki-san,

Thanks for your time over this one :)

On Mon, May 20, 2013 at 08:55:33PM -0400, KOSAKI Motohiro wrote:
> (5/20/13 8:04 PM), Rafael Aquini wrote:
> > Intruduce a new flag to make page-cluster fine-grained discards while swapping
> > conditional, as they can be considered detrimental to some setups. However,
> > keep allowing batched discards at sys_swapon() time, when enabled by the
> > system administrator. 
> > 
> > Signed-off-by: Rafael Aquini <aquini@...hat.com>
> > ---
> >  include/linux/swap.h |  8 +++++---
> >  mm/swapfile.c        | 12 ++++++++----
> >  2 files changed, 13 insertions(+), 7 deletions(-)
> > 
> > diff --git a/include/linux/swap.h b/include/linux/swap.h
> > index 1701ce4..ab2e742 100644
> > --- a/include/linux/swap.h
> > +++ b/include/linux/swap.h
> > @@ -19,10 +19,11 @@ struct bio;
> >  #define SWAP_FLAG_PREFER	0x8000	/* set if swap priority specified */
> >  #define SWAP_FLAG_PRIO_MASK	0x7fff
> >  #define SWAP_FLAG_PRIO_SHIFT	0
> > -#define SWAP_FLAG_DISCARD	0x10000 /* discard swap cluster after use */
> > +#define SWAP_FLAG_DISCARD	0x10000 /* enable discard for swap areas */
> > +#define SWAP_FLAG_DISCARD_CLUSTER 0x20000 /* discard swap clusters after use */
> 
> From point of backward compatibility view, 0x10000 should be disable both discarding
> when mount and when IO.

I think you mean 0x10000 should be enable both here, then. That's a nice catch. I'll
try to think a way to accomplish it in a simple fashion.


> And, introducing new two flags, enable mount time discard and enable IO time discard.
> 
> IOW, Please consider newer kernel and older swapon(8) conbination.
> Other than that, looks good to me.
> 
> 
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