Message-ID: <20110827173421.GA2967@redhat.com>
Date:	Sat, 27 Aug 2011 19:34:21 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Michel Lespinasse <walken@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Hugh Dickins <hughd@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Johannes Weiner <jweiner@...hat.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Shaohua Li <shaohua.li@...el.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH] thp: tail page refcounting fix #4

On Sat, Aug 27, 2011 at 02:41:52AM -0700, Michel Lespinasse wrote:
> I understand you may have to remove the VM_BUG_ON(page_mapcount(page) <= 0)
> that I had suggested in __get_page_tail() (sorry about that).

The function doing that is snd_pcm_mmap_data_fault and it's doing what
I described in prev email.

> My only additional suggestion is about the put_page_testzero in
> __get_page_tail(), maybe if you could just increment the tail page count
> instead of calling __get_page_tail_foll(), then you wouldn't have to
> release the extra head page count there. And it would even look kinda
> natural, head page count gets acquired before compound_lock_irqsave(),
> so we only have to acquire an extra tail page count after confirming
> this is still a tail page.

Ok, I added a param to __get_page_tail_foll, it is constant at build
time and because it's inline the branch should be optimized away at
build time without requiring a separate function. The bugchecks are
the same so we can share and just skip the atomic_inc on the
page_head in __get_page_tail_foll. Also it had to be moved into
internal.h as a further cleanup.

> Either way, the code looks OK by now.
> 
> Reviewed-by: Michel Lespinasse <walken@...gle.com>

Thanks. Incremental diff to correct the false positive bug on for
drivers like alsa allocating __GFP_COMP and mapping subpages with page
faults.

diff --git a/mm/swap.c b/mm/swap.c
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -166,12 +166,6 @@ int __get_page_tail(struct page *page)
 		flags = compound_lock_irqsave(page_head);
 		/* here __split_huge_page_refcount won't run anymore */
 		if (likely(PageTail(page))) {
-			/*
-			 * get_page() can only be called on tail pages
-			 * after get_page_foll() taken a tail page
-			 * refcount.
-			 */
-			VM_BUG_ON(page_mapcount(page) <= 0);
 			__get_page_tail_foll(page);
 			got = 1;

 			/*

This is the optimization.

diff --git a/include/linux/mm.h b/include/linux/mm.h
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -375,26 +375,6 @@ static inline int page_count(struct page
 	return atomic_read(&compound_head(page)->_count);
 }
 
-static inline void __get_page_tail_foll(struct page *page)
-{
-	/*
-	 * If we're getting a tail page, the elevated page->_count is
-	 * required only in the head page and we will elevate the head
-	 * page->_count and tail page->_mapcount.
-	 *
-	 * We elevate page_tail->_mapcount for tail pages to force
-	 * page_tail->_count to be zero at all times to avoid getting
-	 * false positives from get_page_unless_zero() with
-	 * speculative page access (like in
-	 * page_cache_get_speculative()) on tail pages.
-	 */
-	VM_BUG_ON(atomic_read(&page->first_page->_count) <= 0);
-	VM_BUG_ON(atomic_read(&page->_count) != 0);
-	VM_BUG_ON(page_mapcount(page) < 0);
-	atomic_inc(&page->first_page->_count);
-	atomic_inc(&page->_mapcount);
-}
-
 extern int __get_page_tail(struct page *page);
 
 static inline void get_page(struct page *page)
diff --git a/mm/internal.h b/mm/internal.h
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -37,6 +37,28 @@ static inline void __put_page(struct pag
 	atomic_dec(&page->_count);
 }
 
+static inline void __get_page_tail_foll(struct page *page,
+					bool get_page_head)
+{
+	/*
+	 * If we're getting a tail page, the elevated page->_count is
+	 * required only in the head page and we will elevate the head
+	 * page->_count and tail page->_mapcount.
+	 *
+	 * We elevate page_tail->_mapcount for tail pages to force
+	 * page_tail->_count to be zero at all times to avoid getting
+	 * false positives from get_page_unless_zero() with
+	 * speculative page access (like in
+	 * page_cache_get_speculative()) on tail pages.
+	 */
+	VM_BUG_ON(atomic_read(&page->first_page->_count) <= 0);
+	VM_BUG_ON(atomic_read(&page->_count) != 0);
+	VM_BUG_ON(page_mapcount(page) < 0);
+	if (get_page_head)
+		atomic_inc(&page->first_page->_count);
+	atomic_inc(&page->_mapcount);
+}
+
 static inline void get_page_foll(struct page *page)
 {
 	if (unlikely(PageTail(page)))
@@ -45,7 +67,7 @@ static inline void get_page_foll(struct 
 		 * __split_huge_page_refcount() can't run under
 		 * get_page_foll() because we hold the proper PT lock.
 		 */
-		__get_page_tail_foll(page);
+		__get_page_tail_foll(page, true);
 	else {
 		/*
 		 * Getting a normal page or the head of a compound page
diff --git a/mm/swap.c b/mm/swap.c
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -166,16 +166,8 @@ int __get_page_tail(struct page *page)
 		flags = compound_lock_irqsave(page_head);
 		/* here __split_huge_page_refcount won't run anymore */
 		if (likely(PageTail(page))) {
-			__get_page_tail_foll(page);
+			__get_page_tail_foll(page, false);
 			got = 1;
-			/*
-			 * 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);
 		}
 		compound_unlock_irqrestore(page_head, flags);
 		if (unlikely(!got))


===
Subject: thp: tail page refcounting fix

From: Andrea Arcangeli <aarcange@...hat.com>

Michel while working on the working set estimation code, noticed that calling
get_page_unless_zero() on a random pfn_to_page(random_pfn) wasn't safe, if the
pfn ended up being a tail page of a transparent hugepage under splitting by
__split_huge_page_refcount(). He then found the problem could also
theoretically materialize with page_cache_get_speculative() during the
speculative radix tree lookups that uses get_page_unless_zero() in SMP if the
radix tree page is freed and reallocated and get_user_pages is called on it
before page_cache_get_speculative has a chance to call get_page_unless_zero().

So the best way to fix the problem is to keep page_tail->_count zero at all
times. This will guarantee that get_page_unless_zero() can never succeed on any
tail page. page_tail->_mapcount is guaranteed zero and is unused for all tail
pages of a compound page, so we can simply account the tail page references
there and transfer them to tail_page->_count in __split_huge_page_refcount() (in
addition to the head_page->_mapcount).

While debugging this s/_count/_mapcount/ change I also noticed get_page is
called by direct-io.c on pages returned by get_user_pages. That wasn't entirely
safe because the two atomic_inc in get_page weren't atomic. As opposed other
get_user_page users like secondary-MMU page fault to establish the shadow
pagetables would never call any superflous get_page after get_user_page
returns. It's safer to make get_page universally safe for tail pages and to use
get_page_foll() within follow_page (inside get_user_pages()). get_page_foll()
is safe to do the refcounting for tail pages without taking any locks because
it is run within PT lock protected critical sections (PT lock for pte and
page_table_lock for pmd_trans_huge). The standard get_page() as invoked by
direct-io instead will now take the compound_lock but still only for tail
pages. The direct-io paths are usually I/O bound and the compound_lock is per
THP so very finegrined, so there's no risk of scalability issues with it. A
simple direct-io benchmarks with all lockdep prove locking and spinlock
debugging infrastructure enabled shows identical performance and no overhead.
So it's worth it. Ideally direct-io should stop calling get_page() on pages
returned by get_user_pages(). The spinlock in get_page() is already optimized
away for no-THP builds but doing get_page() on tail pages returned by GUP is
generally a rare operation and usually only run in I/O paths.

This new refcounting on page_tail->_mapcount in addition to avoiding new RCU
critical sections will also allow the working set estimation code to work
without any further complexity associated to the tail page refcounting
with THP.

Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
Reported-by: Michel Lespinasse <walken@...gle.com>
Reviewed-by: Michel Lespinasse <walken@...gle.com>
---

diff --git a/arch/powerpc/mm/gup.c b/arch/powerpc/mm/gup.c
--- a/arch/powerpc/mm/gup.c
+++ b/arch/powerpc/mm/gup.c
@@ -22,8 +22,9 @@ static inline void get_huge_page_tail(st
 	 * __split_huge_page_refcount() cannot run
 	 * from under us.
 	 */
-	VM_BUG_ON(atomic_read(&page->_count) < 0);
-	atomic_inc(&page->_count);
+	VM_BUG_ON(page_mapcount(page) < 0);
+	VM_BUG_ON(atomic_read(&page->_count) != 0);
+	atomic_inc(&page->_mapcount);
 }
 
 /*
diff --git a/arch/x86/mm/gup.c b/arch/x86/mm/gup.c
--- a/arch/x86/mm/gup.c
+++ b/arch/x86/mm/gup.c
@@ -114,8 +114,9 @@ static inline void get_huge_page_tail(st
 	 * __split_huge_page_refcount() cannot run
 	 * from under us.
 	 */
-	VM_BUG_ON(atomic_read(&page->_count) < 0);
-	atomic_inc(&page->_count);
+	VM_BUG_ON(page_mapcount(page) < 0);
+	VM_BUG_ON(atomic_read(&page->_count) != 0);
+	atomic_inc(&page->_mapcount);
 }
 
 static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr,
diff --git a/include/linux/mm.h b/include/linux/mm.h
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -356,36 +356,39 @@ static inline struct page *compound_head
 	return page;
 }
 
+/*
+ * The atomic page->_mapcount, starts from -1: so that transitions
+ * both from it and to it can be tracked, using atomic_inc_and_test
+ * and atomic_add_negative(-1).
+ */
+static inline void reset_page_mapcount(struct page *page)
+{
+	atomic_set(&(page)->_mapcount, -1);
+}
+
+static inline int page_mapcount(struct page *page)
+{
+	return atomic_read(&(page)->_mapcount) + 1;
+}
+
 static inline int page_count(struct page *page)
 {
 	return atomic_read(&compound_head(page)->_count);
 }
 
+extern int __get_page_tail(struct page *page);
+
 static inline void get_page(struct page *page)
 {
+	if (unlikely(PageTail(page)))
+		if (likely(__get_page_tail(page)))
+			return;
 	/*
 	 * Getting a normal page or the head of a compound page
-	 * requires to already have an elevated page->_count. Only if
-	 * we're getting a tail page, the elevated page->_count is
-	 * required only in the head page, so for tail pages the
-	 * bugcheck only verifies that the page->_count isn't
-	 * negative.
+	 * requires to already have an elevated page->_count.
 	 */
-	VM_BUG_ON(atomic_read(&page->_count) < !PageTail(page));
+	VM_BUG_ON(atomic_read(&page->_count) <= 0);
 	atomic_inc(&page->_count);
-	/*
-	 * Getting a tail page will elevate both the head and tail
-	 * page->_count(s).
-	 */
-	if (unlikely(PageTail(page))) {
-		/*
-		 * This is safe only because
-		 * __split_huge_page_refcount can't run under
-		 * get_page().
-		 */
-		VM_BUG_ON(atomic_read(&page->first_page->_count) <= 0);
-		atomic_inc(&page->first_page->_count);
-	}
 }
 
 static inline struct page *virt_to_head_page(const void *x)
@@ -804,21 +807,6 @@ static inline pgoff_t page_index(struct 
 }
 
 /*
- * The atomic page->_mapcount, like _count, starts from -1:
- * so that transitions both from it and to it can be tracked,
- * using atomic_inc_and_test and atomic_add_negative(-1).
- */
-static inline void reset_page_mapcount(struct page *page)
-{
-	atomic_set(&(page)->_mapcount, -1);
-}
-
-static inline int page_mapcount(struct page *page)
-{
-	return atomic_read(&(page)->_mapcount) + 1;
-}
-
-/*
  * Return true if this page is mapped into pagetables.
  */
 static inline int page_mapped(struct page *page)
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -62,10 +62,23 @@ struct page {
 			struct {
 
 				union {
-					atomic_t _mapcount;	/* Count of ptes mapped in mms,
-							 * to show when page is mapped
-							 * & limit reverse map searches.
-							 */
+					/*
+					 * Count of ptes mapped in
+					 * mms, to show when page is
+					 * mapped & limit reverse map
+					 * searches.
+					 *
+					 * Used also for tail pages
+					 * refcounting instead of
+					 * _count. Tail pages cannot
+					 * be mapped and keeping the
+					 * tail page _count zero at
+					 * all times guarantees
+					 * get_page_unless_zero() will
+					 * never succeed on tail
+					 * pages.
+					 */
+					atomic_t _mapcount;
 
 					struct {
 						unsigned inuse:16;
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -989,7 +989,7 @@ struct page *follow_trans_huge_pmd(struc
 	page += (addr & ~HPAGE_PMD_MASK) >> PAGE_SHIFT;
 	VM_BUG_ON(!PageCompound(page));
 	if (flags & FOLL_GET)
-		get_page(page);
+		get_page_foll(page);
 
 out:
 	return page;
@@ -1156,6 +1156,7 @@ static void __split_huge_page_refcount(s
 	unsigned long head_index = page->index;
 	struct zone *zone = page_zone(page);
 	int zonestat;
+	int tail_count = 0;
 
 	/* prevent PageLRU to go away from under us, and freeze lru stats */
 	spin_lock_irq(&zone->lru_lock);
@@ -1164,11 +1165,14 @@ static void __split_huge_page_refcount(s
 	for (i = 1; i < HPAGE_PMD_NR; i++) {
 		struct page *page_tail = page + i;
 
-		/* tail_page->_count cannot change */
-		atomic_sub(atomic_read(&page_tail->_count), &page->_count);
-		BUG_ON(page_count(page) <= 0);
-		atomic_add(page_mapcount(page) + 1, &page_tail->_count);
-		BUG_ON(atomic_read(&page_tail->_count) <= 0);
+		/* tail_page->_mapcount cannot change */
+		BUG_ON(page_mapcount(page_tail) < 0);
+		tail_count += page_mapcount(page_tail);
+		/* check for overflow */
+		BUG_ON(tail_count < 0);
+		BUG_ON(atomic_read(&page_tail->_count) != 0);
+		atomic_add(page_mapcount(page) + page_mapcount(page_tail) + 1,
+			   &page_tail->_count);
 
 		/* after clearing PageTail the gup refcount can be released */
 		smp_mb();
@@ -1186,10 +1190,7 @@ static void __split_huge_page_refcount(s
 				      (1L << PG_uptodate)));
 		page_tail->flags |= (1L << PG_dirty);
 
-		/*
-		 * 1) clear PageTail before overwriting first_page
-		 * 2) clear PageTail before clearing PageHead for VM_BUG_ON
-		 */
+		/* clear PageTail before overwriting first_page */
 		smp_wmb();
 
 		/*
@@ -1206,7 +1207,6 @@ static void __split_huge_page_refcount(s
 		 * status is achieved setting a reserved bit in the
 		 * pmd, not by clearing the present bit.
 		*/
-		BUG_ON(page_mapcount(page_tail));
 		page_tail->_mapcount = page->_mapcount;
 
 		BUG_ON(page_tail->mapping);
@@ -1223,6 +1223,8 @@ static void __split_huge_page_refcount(s
 
 		lru_add_page_tail(zone, page, page_tail);
 	}
+	atomic_sub(tail_count, &page->_count);
+	BUG_ON(atomic_read(&page->_count) <= 0);
 
 	__dec_zone_page_state(page, NR_ANON_TRANSPARENT_HUGEPAGES);
 	__mod_zone_page_state(zone, NR_ANON_PAGES, HPAGE_PMD_NR);
diff --git a/mm/internal.h b/mm/internal.h
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -37,6 +37,47 @@ static inline void __put_page(struct pag
 	atomic_dec(&page->_count);
 }
 
+static inline void __get_page_tail_foll(struct page *page,
+					bool get_page_head)
+{
+	/*
+	 * If we're getting a tail page, the elevated page->_count is
+	 * required only in the head page and we will elevate the head
+	 * page->_count and tail page->_mapcount.
+	 *
+	 * We elevate page_tail->_mapcount for tail pages to force
+	 * page_tail->_count to be zero at all times to avoid getting
+	 * false positives from get_page_unless_zero() with
+	 * speculative page access (like in
+	 * page_cache_get_speculative()) on tail pages.
+	 */
+	VM_BUG_ON(atomic_read(&page->first_page->_count) <= 0);
+	VM_BUG_ON(atomic_read(&page->_count) != 0);
+	VM_BUG_ON(page_mapcount(page) < 0);
+	if (get_page_head)
+		atomic_inc(&page->first_page->_count);
+	atomic_inc(&page->_mapcount);
+}
+
+static inline void get_page_foll(struct page *page)
+{
+	if (unlikely(PageTail(page)))
+		/*
+		 * This is safe only because
+		 * __split_huge_page_refcount() can't run under
+		 * get_page_foll() because we hold the proper PT lock.
+		 */
+		__get_page_tail_foll(page, true);
+	else {
+		/*
+		 * Getting a normal page or the head of a compound page
+		 * requires to already have an elevated page->_count.
+		 */
+		VM_BUG_ON(atomic_read(&page->_count) <= 0);
+		atomic_inc(&page->_count);
+	}
+}
+
 extern unsigned long highest_memmap_pfn;
 
 /*
diff --git a/mm/memory.c b/mm/memory.c
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1503,7 +1503,7 @@ split_fallthrough:
 	}
 
 	if (flags & FOLL_GET)
-		get_page(page);
+		get_page_foll(page);
 	if (flags & FOLL_TOUCH) {
 		if ((flags & FOLL_WRITE) &&
 		    !pte_dirty(pte) && !PageDirty(page))
diff --git a/mm/swap.c b/mm/swap.c
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -78,39 +78,21 @@ static void put_compound_page(struct pag
 {
 	if (unlikely(PageTail(page))) {
 		/* __split_huge_page_refcount can run under us */
-		struct page *page_head = page->first_page;
-		smp_rmb();
-		/*
-		 * If PageTail is still set after smp_rmb() we can be sure
-		 * that the page->first_page we read wasn't a dangling pointer.
-		 * See __split_huge_page_refcount() smp_wmb().
-		 */
-		if (likely(PageTail(page) && get_page_unless_zero(page_head))) {
+		struct page *page_head = compound_trans_head(page);
+		if (likely(page != page_head &&
+			   get_page_unless_zero(page_head))) {
 			unsigned long flags;
 			/*
-			 * Verify that our page_head wasn't converted
-			 * to a a regular page before we got a
-			 * reference on it.
+			 * 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.
 			 */
-			if (unlikely(!PageHead(page_head))) {
-				/* PageHead is cleared after PageTail */
-				smp_rmb();
-				VM_BUG_ON(PageTail(page));
-				goto out_put_head;
-			}
-			/*
-			 * Only run compound_lock on a valid PageHead,
-			 * after having it pinned with
-			 * get_page_unless_zero() above.
-			 */
-			smp_mb();
-			/* page_head wasn't a dangling pointer */
 			flags = compound_lock_irqsave(page_head);
 			if (unlikely(!PageTail(page))) {
 				/* __split_huge_page_refcount run before us */
 				compound_unlock_irqrestore(page_head, flags);
 				VM_BUG_ON(PageHead(page_head));
-			out_put_head:
 				if (put_page_testzero(page_head))
 					__put_single_page(page_head);
 			out_put_single:
@@ -121,16 +103,17 @@ static void put_compound_page(struct pag
 			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.
+			 * 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(atomic_read(&page->_count) <= 0);
-			atomic_dec(&page->_count);
+			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))
@@ -160,6 +143,40 @@ void put_page(struct page *page)
 }
 EXPORT_SYMBOL(put_page);
 
+int __get_page_tail(struct page *page)
+{
+	/*
+	 * This takes care of get_page() if run on a tail page
+	 * returned by one of the get_user_pages/follow_page variants.
+	 * get_user_pages/follow_page itself doesn't need the compound
+	 * lock because it runs __get_page_tail_foll() under the
+	 * proper PT lock that already serializes against
+	 * split_huge_page().
+	 */
+	unsigned long flags;
+	int got = 0;
+	struct page *page_head = compound_trans_head(page);
+	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 = 1;
+		}
+		compound_unlock_irqrestore(page_head, flags);
+		if (unlikely(!got))
+			put_page(page_head);
+	}
+	return got;
+}
+EXPORT_SYMBOL(__get_page_tail);
+
 /**
  * put_pages_list() - release a list of pages
  * @pages: list of pages threaded on page->lru
--
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