linux-ext4 - Re: [PATCH] resolve duplicate flag no for PG

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Date:	Mon, 14 May 2007 15:55:20 +0800
From:	Fengguang Wu <fengguang.wu@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Rik van Riel <riel@...hat.com>,
	Theodore Ts'o <tytso@....edu>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] resolve duplicate flag no for PG_lazyfree

On Sun, May 13, 2007 at 10:46:30PM -0700, Andrew Morton wrote:
> On Mon, 14 May 2007 10:37:18 +0800 Fengguang Wu <fengguang.wu@...il.com> wrote:
> 
> > PG_lazyfree and PG_booked shares the same bit.
> > 
> > Either it is a bug that shall fixed by the following patch, or
> > the situation should be explicitly documented?
> > 
> > Signed-off-by: Fengguang Wu <wfg@...l.ustc.edu.cn>
> > ---
> >  include/linux/page-flags.h |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > --- linux-2.6.21-mm2.orig/include/linux/page-flags.h
> > +++ linux-2.6.21-mm2/include/linux/page-flags.h
> > @@ -91,7 +91,7 @@
> >  #define PG_buddy		19	/* Page is free, on buddy lists */
> >  #define PG_booked		20	/* Has blocks reserved on-disk */
> >  
> > -#define PG_lazyfree		20	/* MADV_FREE potential throwaway */
> > +#define PG_lazyfree		21	/* MADV_FREE potential throwaway */
> >  
> >  /* PG_owner_priv_1 users should have descriptive aliases */
> >  #define PG_checked		PG_owner_priv_1 /* Used by some filesystems */
> 
> That's an accident: PG_lazyfree got added but the out-of-tree ext4 patches
> didn't get updated.
> 
> otoh, the intersection between pages which are PageBooked() and pages which
> are PageLazyFree() should be zreo, so it'd be good to actually formalise
> this reuse within the ext4 patches.
> 
> otoh2, PageLazyFree() could have reused PG_owner_priv_1.

otoh3: PG_lazyfree and PG_readahead can reuse the same bit, too.
PG_lazyfree applies to anonymous pages, while PG_readahead applies to
file backed pages.

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