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Date: Wed, 20 Nov 2013 18:51:15 +0100
From: Andrea Arcangeli <aarcange@...hat.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
Khalid Aziz <khalid.aziz@...cle.com>,
Pravin Shelar <pshelar@...ira.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Ben Hutchings <bhutchings@...arflare.com>,
Christoph Lameter <cl@...ux.com>,
Johannes Weiner <jweiner@...hat.com>,
Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
Andi Kleen <andi@...stfloor.org>,
Minchan Kim <minchan@...nel.org>,
Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 7/8] mm/swap.c: reorganize put_compound_page()
From: Andrew Morton <akpm@...ux-foundation.org>
Tweak it so save a tab stop, make code layout slightly less nutty.
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
---
mm/swap.c | 254 +++++++++++++++++++++++++++++++-------------------------------
1 file changed, 125 insertions(+), 129 deletions(-)
diff --git a/mm/swap.c b/mm/swap.c
index b4c49bf..ddb470d 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -82,154 +82,150 @@ 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 threat 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 threat
+ * 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;
}
}
--
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