diff options
| -rw-r--r-- | mm/swap.c | 254 |
1 files changed, 125 insertions, 129 deletions
diff --git a/mm/swap.c b/mm/swap.c index 7434e3619c14..d1100b619e61 100644 --- a/mm/swap.c +++ b/mm/swap.c @@ -81,154 +81,150 @@ static void __put_compound_page(struct page *page) static void put_compound_page(struct page *page) { - if (unlikely(PageTail(page))) { - /* __split_huge_page_refcount can run under us */ - struct page *page_head = compound_trans_head(page); + struct page *page_head; - /* - * THP can not break up slab pages so avoid taking - * compound_lock() and skip the tail page refcounting - * (in _mapcount) too. Slab performs non-atomic bit - * ops on page->flags for better performance. In - * particular slab_unlock() in slub used to be a hot - * path. It is still hot on arches that do not support - * this_cpu_cmpxchg_double(). - * - * If "page" is part of a slab or hugetlbfs page it - * cannot be splitted and the head page cannot change - * from under us. And if "page" is part of a THP page - * under splitting, if the head page pointed by the - * THP tail isn't a THP head anymore, we'll find - * PageTail clear after smp_rmb() and we'll treat it - * as a single page. - */ - if (!__compound_tail_refcounted(page_head)) { + if (likely(!PageTail(page))) { + if (put_page_testzero(page)) { /* - * 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. + * By the time all refcounts have been released + * split_huge_page cannot run anymore from under us. */ - smp_rmb(); - if (likely(PageTail(page))) { - /* - * __split_huge_page_refcount - * cannot race here. - */ - VM_BUG_ON(!PageHead(page_head)); - VM_BUG_ON(page_mapcount(page) != 0); - 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(PageSlab(page_head)); - __put_compound_page(page_head); - } - return; - } 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). - */ - goto out_put_single; + if (PageHead(page)) + __put_compound_page(page); + else + __put_single_page(page); } + return; + } - if (likely(page != page_head && - get_page_unless_zero(page_head))) { - unsigned long flags; + /* __split_huge_page_refcount can run under us */ + page_head = compound_trans_head(page); + /* + * THP can not break up slab pages so avoid taking + * compound_lock() and skip the tail page refcounting (in + * _mapcount) too. Slab performs non-atomic bit ops on + * page->flags for better performance. In particular + * slab_unlock() in slub used to be a hot path. It is still + * hot on arches that do not support + * this_cpu_cmpxchg_double(). + * + * If "page" is part of a slab or hugetlbfs page it cannot be + * splitted and the head page cannot change from under us. And + * if "page" is part of a THP page under splitting, if the + * head page pointed by the THP tail isn't a THP head anymore, + * we'll find PageTail clear after smp_rmb() and we'll treat + * it as a single page. + */ + if (!__compound_tail_refcounted(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))) { /* - * page_head wasn't a dangling pointer but it - * may not be a head page anymore by the time - * we obtain the lock. That is ok as long as it - * can't be freed from under us. + * __split_huge_page_refcount cannot race + * here. */ - flags = compound_lock_irqsave(page_head); - if (unlikely(!PageTail(page))) { - /* __split_huge_page_refcount run before us */ - compound_unlock_irqrestore(page_head, flags); - if (put_page_testzero(page_head)) { - /* - * The head page may have been - * freed and reallocated as a - * compound page of smaller - * order and then freed again. - * All we know is that it - * cannot have become: a THP - * page, a compound page of - * higher order, a tail page. - * That is because we still - * hold the refcount of the - * split THP tail and - * page_head was the THP head - * before the split. - */ - if (PageHead(page_head)) - __put_compound_page(page_head); - else - __put_single_page(page_head); - } -out_put_single: - if (put_page_testzero(page)) - __put_single_page(page); - return; + VM_BUG_ON(!PageHead(page_head)); + VM_BUG_ON(page_mapcount(page) != 0); + 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(PageSlab(page_head)); + __put_compound_page(page_head); } - VM_BUG_ON(page_head != page->first_page); + return; + } else /* - * We can release the refcount taken by - * get_page_unless_zero() now that - * __split_huge_page_refcount() is blocked on - * the compound_lock. + * __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). */ - if (put_page_testzero(page_head)) - VM_BUG_ON(1); - /* __split_huge_page_refcount will wait now */ - VM_BUG_ON(page_mapcount(page) <= 0); - atomic_dec(&page->_mapcount); - VM_BUG_ON(atomic_read(&page_head->_count) <= 0); - VM_BUG_ON(atomic_read(&page->_count) != 0); - compound_unlock_irqrestore(page_head, flags); + goto out_put_single; + } + + if (likely(page != page_head && get_page_unless_zero(page_head))) { + unsigned long flags; + /* + * page_head wasn't a dangling pointer but it may not + * be a head page anymore by the time we obtain the + * lock. That is ok as long as it can't be freed from + * under us. + */ + flags = compound_lock_irqsave(page_head); + if (unlikely(!PageTail(page))) { + /* __split_huge_page_refcount run before us */ + compound_unlock_irqrestore(page_head, flags); if (put_page_testzero(page_head)) { + /* + * The head page may have been freed + * and reallocated as a compound page + * of smaller order and then freed + * again. All we know is that it + * cannot have become: a THP page, a + * compound page of higher order, a + * tail page. That is because we + * still hold the refcount of the + * split THP tail and page_head was + * the THP head before the split. + */ if (PageHead(page_head)) __put_compound_page(page_head); else __put_single_page(page_head); } - } else { - /* page_head is a dangling pointer */ - VM_BUG_ON(PageTail(page)); - goto out_put_single; +out_put_single: + if (put_page_testzero(page)) + __put_single_page(page); + return; } - } else if (put_page_testzero(page)) { - if (PageHead(page)) - __put_compound_page(page); - else - __put_single_page(page); + VM_BUG_ON(page_head != page->first_page); + /* + * We can release the refcount taken by + * get_page_unless_zero() now that + * __split_huge_page_refcount() is blocked on the + * compound_lock. + */ + if (put_page_testzero(page_head)) + VM_BUG_ON(1); + /* __split_huge_page_refcount will wait now */ + VM_BUG_ON(page_mapcount(page) <= 0); + atomic_dec(&page->_mapcount); + VM_BUG_ON(atomic_read(&page_head->_count) <= 0); + VM_BUG_ON(atomic_read(&page->_count) != 0); + compound_unlock_irqrestore(page_head, flags); + + if (put_page_testzero(page_head)) { + if (PageHead(page_head)) + __put_compound_page(page_head); + else + __put_single_page(page_head); + } + } else { + /* page_head is a dangling pointer */ + VM_BUG_ON(PageTail(page)); + goto out_put_single; } } |