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Commit c747ce79 authored by Jianyu Zhan's avatar Jianyu Zhan Committed by Linus Torvalds
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mm/swap.c: introduce put_[un]refcounted_compound_page helpers for splitting put_compound_page() 



Currently, put_compound_page() carefully handles tricky cases to avoid
racing with compound page releasing or splitting, which makes it quite
lenthy (about 200+ lines) and needs deep tab indention, which makes it
quite hard to follow and maintain.

This patch and the next patch refactor this function.

Based on the code skeleton of put_compound_page:

put_compound_pge:
        if !PageTail(page)
        	put head page fastpath;
		return;

        /* else PageTail */
        page_head = compound_head(page)
        if !__compound_tail_refcounted(page_head)
		put head page optimal path; <---(1)
		return;
        else
		put head page slowpath; <--- (2)
                return;

This patch introduces two helpers, put_[un]refcounted_compound_page,
handling the code path (1) and code path (2), respectively.  They both are
tagged __always_inline, thus elmiating function call overhead, making them
operating the same way as before.

They are almost copied verbatim(except one place, a "goto out_put_single"
is expanded), with some comments rephrasing.

Signed-off-by: default avatarJianyu Zhan <nasa4836@gmail.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Jiang Liu <liuj97@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 23c8902d

mm/swap.c

+142 −0
Original line number Diff line number Diff line
@@ -79,6 +79,148 @@ static void __put_compound_page(struct page *page) 
	(*dtor)(page);

}



/**

 * Two special cases here: we could avoid taking compound_lock_irqsave

 * and could skip the tail refcounting(in _mapcount).

 *

 * 1. Hugetlbfs page:

 *

 *    PageHeadHuge will remain true until the compound page

 *    is released and enters the buddy allocator, and it could

 *    not be split by __split_huge_page_refcount().

 *

 *    So if we see PageHeadHuge set, and we have the tail page pin,

 *    then we could safely put head page.

 *

 * 2. Slab THP page:

 *

 *    PG_slab is cleared before the slab frees the head page, and

 *    tail pin cannot be the last reference left on the head page,

 *    because the slab code is free to reuse the compound page

 *    after a kfree/kmem_cache_free without having to check if

 *    there's any tail pin left.  In turn all tail pinsmust be always

 *    released while the head is still pinned by the slab code

 *    and so we know PG_slab will be still set too.

 *

 *    So if we see PageSlab set, and we have the tail page pin,

 *    then we could safely put head page.

 */

static __always_inline

void put_unrefcounted_compound_page(struct page *page_head, struct page *page)

{

	/*

	 * If @page is a THP tail, we must read the tail page

	 * flags after the head page flags. The

	 * __split_huge_page_refcount 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, see the comment above this function.

		 */

		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 THP 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.

			 *

			 * If this is the tail of a hugetlbfs page,

			 * 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

		/*

		 * __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))

			__put_single_page(page);

}



static __always_inline

void put_refcounted_compound_page(struct page *page_head, struct page *page)

{

	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 @page_head 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_PAGE(page_head != page->first_page, 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_PAGE(1, page_head);

		/* __split_huge_page_refcount will wait now */

		VM_BUG_ON_PAGE(page_mapcount(page) <= 0, page);

		atomic_dec(&page->_mapcount);

		VM_BUG_ON_PAGE(atomic_read(&page_head->_count) <= 0, page_head);

		VM_BUG_ON_PAGE(atomic_read(&page->_count) != 0, page);

		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_PAGE(PageTail(page), page);

		goto out_put_single;

	}

}



static void put_compound_page(struct page *page)

{

	struct page *page_head;