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    SubjectRe: [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@gmail.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@mail.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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