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    SubjectRe: [PATCH] rmap: add exclusively owned pages to the newest anon_vma


    On Wed, 14 Apr 2010, Rik van Riel wrote:
    > - /*
    > - * 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;
    > + }

    I really dislike your coding style.

    If we do this conditionally, we're _much_ better off declaring the
    variables we only use inside that conditional block inside the block
    itself. And since we access "vma->anon_vma" in either case, just move that
    case outside the conditional statement, and avoid a pointless
    if/then/else.

    IOW, something like this. Totally untested.

    Linus

    ---
    mm/rmap.c | 26 +++++++++++++++-----------
    1 files changed, 15 insertions(+), 11 deletions(-)

    diff --git a/mm/rmap.c b/mm/rmap.c
    index 4bad326..78d4730 100644
    --- a/mm/rmap.c
    +++ b/mm/rmap.c
    @@ -732,21 +732,25 @@ void page_move_anon_rmap(struct page *page,
    * @address: the user virtual address mapped
    */
    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;
    + struct anon_vma *anon_vma = vma->anon_vma;

    - BUG_ON(!vma->anon_vma);
    + BUG_ON(!anon_vma);

    /*
    - * We must use the _oldest_ possible anon_vma for the page mapping!
    + * If the page isn't exclusively mapped into this 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.
    + * 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) {
    + struct anon_vma_chain *avc;
    + 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 +806,7 @@ 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 +828,7 @@ 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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