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    Subject[PATCH 1/3] compound page: use page[1].lru
    If a compound page has its own put_page_testzero destructor (the only
    current example is free_huge_page), that is noted in page[1].mapping of
    the compound page. But that's rather a poor place to keep it: functions
    which call set_page_dirty_lock after get_user_pages (e.g. Infiniband's
    __ib_umem_release) ought to be checking first, otherwise set_page_dirty
    is liable to crash on what's not the address of a struct address_space.

    And now I'm about to make that worse: it turns out that every compound
    page needs a destructor, so we can no longer rely on hugetlb pages going
    their own special way, to avoid further problems of page->mapping reuse.
    For example, not many people know that: on 50% of i386 -Os builds, the
    first tail page of a compound page purports to be PageAnon (when its
    destructor has an odd address), which surprises page_add_file_rmap.

    Keep the compound page destructor in page[1].lru.next instead. And to
    free up the common pairing of mapping and index, also move compound page
    order from index to lru.prev. Slab reuses page->lru too: but if we ever
    need slab to use compound pages, it can easily stack its use above this.

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

    mm/hugetlb.c | 4 ++--
    mm/page_alloc.c | 15 ++++++---------
    mm/swap.c | 2 +-
    3 files changed, 9 insertions(+), 12 deletions(-)

    --- 2.6.16-rc2-git11/mm/hugetlb.c 2006-02-10 12:28:04.000000000 +0000
    +++ 2.6.16-rc2-git11+/mm/hugetlb.c 2006-02-10 19:54:44.000000000 +0000
    @@ -85,7 +85,7 @@ void free_huge_page(struct page *page)
    BUG_ON(page_count(page));

    INIT_LIST_HEAD(&page->lru);
    - page[1].mapping = NULL;
    + page[1].lru.next = NULL; /* reset dtor */

    spin_lock(&hugetlb_lock);
    enqueue_huge_page(page);
    @@ -105,7 +105,7 @@ struct page *alloc_huge_page(struct vm_a
    }
    spin_unlock(&hugetlb_lock);
    set_page_count(page, 1);
    - page[1].mapping = (void *)free_huge_page;
    + page[1].lru.next = (void *)free_huge_page; /* set dtor */
    for (i = 0; i < (HPAGE_SIZE/PAGE_SIZE); ++i)
    clear_user_highpage(&page[i], addr);
    return page;
    --- 2.6.16-rc2-git11/mm/page_alloc.c 2006-02-10 12:28:04.000000000 +0000
    +++ 2.6.16-rc2-git11+/mm/page_alloc.c 2006-02-10 20:03:19.000000000 +0000
    @@ -169,20 +169,17 @@ static void bad_page(struct page *page)
    * All pages have PG_compound set. All pages have their ->private pointing at
    * the head page (even the head page has this).
    *
    - * The first tail page's ->mapping, if non-zero, holds the address of the
    - * compound page's put_page() function.
    - *
    - * The order of the allocation is stored in the first tail page's ->index
    - * This is only for debug at present. This usage means that zero-order pages
    - * may not be compound.
    + * The first tail page's ->lru.next holds the address of the compound page's
    + * put_page() function. Its ->lru.prev holds the order of allocation.
    + * This usage means that zero-order pages may not be compound.
    */
    static void prep_compound_page(struct page *page, unsigned long order)
    {
    int i;
    int nr_pages = 1 << order;

    - page[1].mapping = NULL;
    - page[1].index = order;
    + page[1].lru.next = NULL; /* set dtor */
    + page[1].lru.prev = (void *)order;
    for (i = 0; i < nr_pages; i++) {
    struct page *p = page + i;

    @@ -196,7 +193,7 @@ static void destroy_compound_page(struct
    int i;
    int nr_pages = 1 << order;

    - if (unlikely(page[1].index != order))
    + if (unlikely((unsigned long)page[1].lru.prev != order))
    bad_page(page);

    for (i = 0; i < nr_pages; i++) {
    --- 2.6.16-rc2-git11/mm/swap.c 2006-02-10 12:28:04.000000000 +0000
    +++ 2.6.16-rc2-git11+/mm/swap.c 2006-02-10 19:54:44.000000000 +0000
    @@ -40,7 +40,7 @@ static void put_compound_page(struct pag
    if (put_page_testzero(page)) {
    void (*dtor)(struct page *page);

    - dtor = (void (*)(struct page *))page[1].mapping;
    + dtor = (void (*)(struct page *))page[1].lru.next;
    (*dtor)(page);
    }
    }
    -
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