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    Subject[PATCH 04/32] mm: introduce PG_readahead
    Introduce a new page flag: PG_readahead.

    It acts as a look-ahead mark, which tells the page reader:
    Hey, it's time to invoke the adaptive read-ahead logic!
    For the sake of I/O pipelining, don't wait until it runs out of
    cached pages. ;-)

    Signed-off-by: Wu Fengguang <wfg@mail.ustc.edu.cn>
    ---

    include/linux/page-flags.h | 6 ++++++
    mm/page_alloc.c | 2 +-
    2 files changed, 7 insertions(+), 1 deletion(-)

    --- linux-2.6.17-rc4-mm3.orig/include/linux/page-flags.h
    +++ linux-2.6.17-rc4-mm3/include/linux/page-flags.h
    @@ -90,6 +90,8 @@
    #define PG_nosave_free 18 /* Free, should not be written */
    #define PG_buddy 19 /* Page is free, on buddy lists */

    +#define PG_readahead 20 /* Reminder to do readahead */
    +

    #if (BITS_PER_LONG > 32)
    /*
    @@ -372,6 +374,10 @@ extern void __mod_page_state_offset(unsi
    #define SetPageUncached(page) set_bit(PG_uncached, &(page)->flags)
    #define ClearPageUncached(page) clear_bit(PG_uncached, &(page)->flags)

    +#define PageReadahead(page) test_bit(PG_readahead, &(page)->flags)
    +#define SetPageReadahead(page) set_bit(PG_readahead, &(page)->flags)
    +#define TestClearPageReadahead(page) test_and_clear_bit(PG_readahead, &(page)->flags)
    +
    struct page; /* forward declaration */

    int test_clear_page_dirty(struct page *page);
    --- linux-2.6.17-rc4-mm3.orig/mm/page_alloc.c
    +++ linux-2.6.17-rc4-mm3/mm/page_alloc.c
    @@ -564,7 +564,7 @@ static int prep_new_page(struct page *pa
    if (PageReserved(page))
    return 1;

    - page->flags &= ~(1 << PG_uptodate | 1 << PG_error |
    + page->flags &= ~(1 << PG_uptodate | 1 << PG_error | 1 << PG_readahead |
    1 << PG_referenced | 1 << PG_arch_1 |
    1 << PG_checked | 1 << PG_mappedtodisk);
    set_page_private(page, 0);
    --
    -
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