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    SubjectRe: [PATCHv4 20/24] mm, thp: remove compound_lock
    Date

    ....
    ....

    static void put_compound_page(struct page *page)
    > {
    > struct page *page_head;
    > - unsigned long flags;
    >
    > if (likely(!PageTail(page))) {
    > if (put_page_testzero(page)) {
    > @@ -108,58 +101,33 @@ static void put_compound_page(struct page *page)
    > /* __split_huge_page_refcount can run under us */
    > page_head = compound_head(page);
    >
    > - if (!compound_lock_needed(page_head)) {
    > - /*
    > - * If "page" is a THP tail, we must read the tail page flags
    > - * after the head page flags. The split_huge_page side enforces
    > - * write memory barriers between clearing PageTail and before
    > - * the head page can be freed and reallocated.
    > - */
    > - smp_rmb();
    > - if (likely(PageTail(page))) {
    > - /* __split_huge_page_refcount cannot race here. */
    > - VM_BUG_ON_PAGE(!PageHead(page_head), page_head);
    > - VM_BUG_ON_PAGE(page_mapcount(page) != 0, page);
    > - if (put_page_testzero(page_head)) {
    > - /*
    > - * If this is the tail of a slab compound page,
    > - * the tail pin must not be the last reference
    > - * held on the page, because the PG_slab cannot
    > - * be cleared before all tail pins (which skips
    > - * the _mapcount tail refcounting) have been
    > - * released. For hugetlbfs the tail pin may be
    > - * the last reference on the page instead,
    > - * because PageHeadHuge will not go away until
    > - * the compound page enters the buddy
    > - * allocator.
    > - */
    > - VM_BUG_ON_PAGE(PageSlab(page_head), page_head);
    > - __put_compound_page(page_head);
    > - }
    > - } else if (put_page_testzero(page))
    > - __put_single_page(page);
    > - return;
    > - }
    > -
    > - flags = compound_lock_irqsave(page_head);
    > - /* here __split_huge_page_refcount won't run anymore */
    > - if (likely(page != page_head && PageTail(page))) {
    > - bool free;
    > -
    > - free = put_page_testzero(page_head);
    > - compound_unlock_irqrestore(page_head, flags);
    > - if (free) {
    > - if (PageHead(page_head))
    > - __put_compound_page(page_head);
    > - else
    > - __put_single_page(page_head);
    > + /*
    > + * If "page" is a THP tail, we must read the tail page flags after the
    > + * head page flags. The split_huge_page side enforces write memory
    > + * barriers between clearing PageTail and before the head page can be
    > + * freed and reallocated.
    > + */
    > + smp_rmb();
    > + if (likely(PageTail(page))) {
    > + /* __split_huge_page_refcount cannot race here. */
    > + VM_BUG_ON_PAGE(!PageHead(page_head), page_head);
    > + if (put_page_testzero(page_head)) {
    > + /*
    > + * If this is the tail of a slab compound page, the
    > + * tail pin must not be the last reference held on the
    > + * page, because the PG_slab cannot be cleared before
    > + * all tail pins (which skips the _mapcount tail
    > + * refcounting) have been released. For hugetlbfs the
    > + * tail pin may be the last reference on the page
    > + * instead, because PageHeadHuge will not go away until
    > + * the compound page enters the buddy allocator.
    > + */
    > + VM_BUG_ON_PAGE(PageSlab(page_head), page_head);
    > + __put_compound_page(page_head);
    > }


    The comment may need an update ? For THP also a tail pin may be the last
    reference on the page right ?


    > - } else {
    > - compound_unlock_irqrestore(page_head, flags);
    > - VM_BUG_ON_PAGE(PageTail(page), page);
    > - if (put_page_testzero(page))
    > - __put_single_page(page);
    > - }
    > + } else if (put_page_testzero(page))
    > + __put_single_page(page);
    > + return;
    > }
    >
    > void put_page(struct page *page)
    > @@ -178,42 +146,29 @@ EXPORT_SYMBOL(put_page);
    > void __get_page_tail(struct page *page)
    > {
    > struct page *page_head = compound_head(page);

    .....
    ....

    > + smp_rmb();
    > + if (likely(PageTail(page))) {
    > + /*
    > + * This is a hugetlbfs page or a slab page.
    > + * __split_huge_page_refcount cannot race here.
    > + */

    This comment also need an update. This can be a THP tail page right ?

    > + VM_BUG_ON_PAGE(!PageHead(page_head), page_head);
    > + VM_BUG_ON(page_head != page->first_page);
    > + VM_BUG_ON_PAGE(atomic_read(&page_head->_count) <= 0,
    > + page);
    > + atomic_inc(&page_head->_count);
    > + } else {
    > + /*
    > + * __split_huge_page_refcount run before us, "page" was
    > + * a thp tail. the split page_head has been freed and
    > + * reallocated as slab or hugetlbfs page of smaller
    > + * order (only possible if reallocated as slab on x86).
    > + */
    > VM_BUG_ON_PAGE(atomic_read(&page->_count) <= 0, page);
    > atomic_inc(&page->_count);
    > }
    > - compound_unlock_irqrestore(page_head, flags);
    > + return;
    > }
    > EXPORT_SYMBOL(__get_page_tail);
    >
    > --
    > 2.1.4



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