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    Subject[PATCH 2/8] badpage: keep any bad page out of circulation
    Until now the bad_page() checkers have special-cased PageReserved, keeping
    those pages out of circulation thereafter. Now extend the special case to
    all: we want to keep ANY page with bad state out of circulation - the
    "free" page may well be in use by something.

    Leave the bad state of those pages untouched, for examination by debuggers;
    except for PageBuddy - leaving that set would risk bringing the page back.

    Signed-off-by: Hugh Dickins <hugh@veritas.com>
    ---

    mm/page_alloc.c | 52 +++++++++++++++++++++-------------------------
    1 file changed, 24 insertions(+), 28 deletions(-)

    --- badpage1/mm/page_alloc.c 2008-11-28 20:40:33.000000000 +0000
    +++ badpage2/mm/page_alloc.c 2008-11-28 20:40:36.000000000 +0000
    @@ -231,9 +231,9 @@ static void bad_page(struct page *page)
    printk(KERN_EMERG "Trying to fix it up, but a reboot is needed\n"
    KERN_EMERG "Backtrace:\n");
    dump_stack();
    - set_page_count(page, 0);
    - reset_page_mapcount(page);
    - page->mapping = NULL;
    +
    + /* Leave bad fields for debug, except PageBuddy could make trouble */
    + __ClearPageBuddy(page);
    add_taint(TAINT_BAD_PAGE);
    }

    @@ -290,25 +290,31 @@ void prep_compound_gigantic_page(struct
    }
    #endif

    -static void destroy_compound_page(struct page *page, unsigned long order)
    +static int destroy_compound_page(struct page *page, unsigned long order)
    {
    int i;
    int nr_pages = 1 << order;
    + int bad = 0;

    - if (unlikely(compound_order(page) != order))
    + if (unlikely(compound_order(page) != order) ||
    + unlikely(!PageHead(page))) {
    bad_page(page);
    + bad++;
    + }

    - if (unlikely(!PageHead(page)))
    - bad_page(page);
    __ClearPageHead(page);
    +
    for (i = 1; i < nr_pages; i++) {
    struct page *p = page + i;

    - if (unlikely(!PageTail(p) |
    - (p->first_page != page)))
    + if (unlikely(!PageTail(p) | (p->first_page != page))) {
    bad_page(page);
    + bad++;
    + }
    __ClearPageTail(p);
    }
    +
    + return bad;
    }

    static inline void prep_zero_page(struct page *page, int order, gfp_t gfp_flags)
    @@ -428,7 +434,8 @@ static inline void __free_one_page(struc
    int migratetype = get_pageblock_migratetype(page);

    if (unlikely(PageCompound(page)))
    - destroy_compound_page(page, order);
    + if (unlikely(destroy_compound_page(page, order)))
    + return;

    page_idx = page_to_pfn(page) & ((1 << MAX_ORDER) - 1);

    @@ -465,15 +472,10 @@ static inline int free_pages_check(struc
    if (unlikely(page_mapcount(page) |
    (page->mapping != NULL) |
    (page_count(page) != 0) |
    - (page->flags & PAGE_FLAGS_CHECK_AT_FREE)))
    + (page->flags & PAGE_FLAGS_CHECK_AT_FREE))) {
    bad_page(page);
    - /*
    - * For now, we report if PG_reserved was found set, but do not
    - * clear it, and do not free the page. But we shall soon need
    - * to do more, for when the ZERO_PAGE count wraps negative.
    - */
    - if (PageReserved(page))
    return 1;
    + }
    if (page->flags & PAGE_FLAGS_CHECK_AT_PREP)
    page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
    return 0;
    @@ -521,11 +523,11 @@ static void __free_pages_ok(struct page
    {
    unsigned long flags;
    int i;
    - int reserved = 0;
    + int bad = 0;

    for (i = 0 ; i < (1 << order) ; ++i)
    - reserved += free_pages_check(page + i);
    - if (reserved)
    + bad += free_pages_check(page + i);
    + if (bad)
    return;

    if (!PageHighMem(page)) {
    @@ -610,17 +612,11 @@ static int prep_new_page(struct page *pa
    if (unlikely(page_mapcount(page) |
    (page->mapping != NULL) |
    (page_count(page) != 0) |
    - (page->flags & PAGE_FLAGS_CHECK_AT_PREP)))
    + (page->flags & PAGE_FLAGS_CHECK_AT_PREP))) {
    bad_page(page);
    -
    - /*
    - * For now, we report if PG_reserved was found set, but do not
    - * clear it, and do not allocate the page: as a safety net.
    - */
    - if (PageReserved(page))
    return 1;
    + }

    - page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
    set_page_private(page, 0);
    set_page_refcounted(page);


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