diff options
Diffstat (limited to 'mm/swap.c')
| -rw-r--r-- | mm/swap.c | 285 |
1 files changed, 170 insertions, 115 deletions
diff --git a/mm/swap.c b/mm/swap.c index 7a9f80d451f5..d1100b619e61 100644 --- a/mm/swap.c +++ b/mm/swap.c @@ -31,7 +31,6 @@ #include <linux/memcontrol.h> #include <linux/gfp.h> #include <linux/uio.h> -#include <linux/hugetlb.h> #include "internal.h" @@ -82,96 +81,150 @@ static void __put_compound_page(struct page *page) static void put_compound_page(struct page *page) { - /* - * hugetlbfs pages cannot be split from under us. If this is a - * hugetlbfs page, check refcount on head page and release the page if - * the refcount becomes zero. - */ - if (PageHuge(page)) { - page = compound_head(page); - if (put_page_testzero(page)) - __put_compound_page(page); + struct page *page_head; + if (likely(!PageTail(page))) { + if (put_page_testzero(page)) { + /* + * By the time all refcounts have been released + * split_huge_page cannot run anymore from under us. + */ + if (PageHead(page)) + __put_compound_page(page); + else + __put_single_page(page); + } return; } - if (unlikely(PageTail(page))) { - /* __split_huge_page_refcount can run under us */ - struct page *page_head = compound_trans_head(page); - - 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))) { /* - * THP can not break up slab pages so avoid taking - * compound_lock(). 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 (PageSlab(page_head)) { - if (PageTail(page)) { - if (put_page_testzero(page_head)) - VM_BUG_ON(1); - - atomic_dec(&page->_mapcount); - goto skip_lock_tail; - } else - goto skip_lock; - } - /* - * 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); -skip_lock: - if (put_page_testzero(page_head)) - __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; + } -skip_lock_tail: + 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; } } @@ -198,51 +251,53 @@ bool __get_page_tail(struct page *page) * proper PT lock that already serializes against * split_huge_page(). */ - bool got = false; - struct page *page_head; - - /* - * If this is a hugetlbfs page it cannot be split under us. Simply - * increment refcount for the head page. - */ - if (PageHuge(page)) { - page_head = compound_head(page); - atomic_inc(&page_head->_count); - got = true; - } else { - unsigned long flags; - - page_head = compound_trans_head(page); - if (likely(page != page_head && - get_page_unless_zero(page_head))) { - - /* Ref to put_compound_page() comment. */ - if (PageSlab(page_head)) { - if (likely(PageTail(page))) { - __get_page_tail_foll(page, false); - return true; - } else { - put_page(page_head); - return false; - } - } - + unsigned long flags; + bool got; + struct page *page_head = compound_trans_head(page); + + /* Ref to put_compound_page() comment. */ + if (!__compound_tail_refcounted(page_head)) { + 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. + * This is a hugetlbfs page or a slab + * page. __split_huge_page_refcount + * cannot race here. */ - flags = compound_lock_irqsave(page_head); - /* here __split_huge_page_refcount won't run anymore */ - if (likely(PageTail(page))) { - __get_page_tail_foll(page, false); - got = true; - } - compound_unlock_irqrestore(page_head, flags); - if (unlikely(!got)) - put_page(page_head); + VM_BUG_ON(!PageHead(page_head)); + __get_page_tail_foll(page, true); + return true; + } 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). + */ + return false; + } + } + + got = false; + if (likely(page != page_head && get_page_unless_zero(page_head))) { + /* + * 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); + /* here __split_huge_page_refcount won't run anymore */ + if (likely(PageTail(page))) { + __get_page_tail_foll(page, false); + got = true; } + compound_unlock_irqrestore(page_head, flags); + if (unlikely(!got)) + put_page(page_head); } return got; } |