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    Subject[PATCH] rmap: add exclusively owned pages to the newest anon_vma
    The recent anon_vma fixes cause many anonymous pages to end up
    in the parent process anon_vma, even when the page is exclusively
    owned by the current process.

    Adding exclusively owned anonymous pages to the top anon_vma
    reduces rmap scanning overhead, especially in workloads with
    forking servers.

    This patch adds a parameter to __page_set_anon_rmap that can
    be used to indicate whether or not the added page is exclusively
    owned by the current process.

    Pages added through page_add_new_anon_rmap are exclusively
    owned by the current process, and can be added to the top
    anon_vma.

    Pages added through page_add_anon_rmap can be either shared
    or exclusively owned, so we do the conservative thing and
    add it to the oldest anon_vma.

    A next step would be to add the exclusive parameter to
    page_add_anon_rmap, to be used from functions where we do
    know for sure whether a page is exclusively owned.

    Signed-off-by: Rik van Riel <riel@redhat.com>
    ---
    Borislav, I audited the code before making this change, but would
    still appreciate your testing of this patch :)

    Linus, once this patch survives Borislav's testing, I'll start
    looking at the next step. I'd like to do things one step at a
    time so I won't cause another regression...

    mm/rmap.c | 30 +++++++++++++++++++-----------
    1 files changed, 19 insertions(+), 11 deletions(-)

    diff --git a/mm/rmap.c b/mm/rmap.c
    index 4bad326..12ac0f1 100644
    --- a/mm/rmap.c
    +++ b/mm/rmap.c
    @@ -730,23 +730,31 @@ void page_move_anon_rmap(struct page *page,
    * @page: the page to add the mapping to
    * @vma: the vm area in which the mapping is added
    * @address: the user virtual address mapped
    + * @exclusive: the page is exclusively owned by the current process
    */
    static void __page_set_anon_rmap(struct page *page,
    - struct vm_area_struct *vma, unsigned long address)
    + struct vm_area_struct *vma, unsigned long address, int exclusive)
    {
    struct anon_vma_chain *avc;
    struct anon_vma *anon_vma;

    BUG_ON(!vma->anon_vma);

    - /*
    - * We must use the _oldest_ possible anon_vma for the page mapping!
    - *
    - * So take the last AVC chain entry in the vma, which is the deepest
    - * ancestor, and use the anon_vma from that.
    - */
    - avc = list_entry(vma->anon_vma_chain.prev, struct anon_vma_chain, same_vma);
    - anon_vma = avc->anon_vma;
    + if (exclusive)
    + anon_vma = vma->anon_vma;
    + else {
    + /*
    + * The page may be shared between multiple processes.
    + * We must use the _oldest_ possible anon_vma for the
    + * page mapping! That anon_vma is guaranteed to be
    + * present in all processes that could share this page.
    + *
    + * So take the last AVC chain entry in the vma, which is the
    + * deepest ancestor, and use the anon_vma from that.
    + */
    + avc = list_entry(vma->anon_vma_chain.prev, struct anon_vma_chain, same_vma);
    + anon_vma = avc->anon_vma;
    + }

    anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
    page->mapping = (struct address_space *) anon_vma;
    @@ -802,7 +810,7 @@ void page_add_anon_rmap(struct page *page,
    VM_BUG_ON(!PageLocked(page));
    VM_BUG_ON(address < vma->vm_start || address >= vma->vm_end);
    if (first)
    - __page_set_anon_rmap(page, vma, address);
    + __page_set_anon_rmap(page, vma, address, 0);
    else
    __page_check_anon_rmap(page, vma, address);
    }
    @@ -824,7 +832,7 @@ void page_add_new_anon_rmap(struct page *page,
    SetPageSwapBacked(page);
    atomic_set(&page->_mapcount, 0); /* increment count (starts at -1) */
    __inc_zone_page_state(page, NR_ANON_PAGES);
    - __page_set_anon_rmap(page, vma, address);
    + __page_set_anon_rmap(page, vma, address, 1);
    if (page_evictable(page, vma))
    lru_cache_add_lru(page, LRU_ACTIVE_ANON);
    else


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