[PATCH 0/2] Fix compound_head() race

[PATCH 0/2] Fix compound_head() race

[Kirill A. Shutemov]

Here's my attempt on fixing recently discovered race in compound_head().
It should make compound_head() reliable in all contexts.

The patchset is against Linus' tree. Let me know if it need to be rebased
onto different baseline.

It's expected to have conflicts with my page-flags patchset and probably
should be applied before it.

Kirill A. Shutemov (2):
  zsmalloc: use page->private instead of page->first_page
  mm: make compound_head() robust

 Documentation/vm/split_page_table_lock |  4 +-
 arch/xtensa/configs/iss_defconfig      |  1 -
 include/linux/mm.h                     | 53 ++--------------------
 include/linux/mm_types.h               | 15 ++++---
 include/linux/page-flags.h             | 80 ++++++++--------------------------
 mm/Kconfig                             | 12 -----
 mm/debug.c                             |  7 ---
 mm/hugetlb.c                           |  8 +---
 mm/internal.h                          |  4 +-
 mm/memory-failure.c                    |  7 ---
 mm/page_alloc.c                        | 36 +++++++--------
 mm/swap.c                              |  4 +-
 mm/zsmalloc.c                          | 11 +++--
 13 files changed, 61 insertions(+), 181 deletions(-)

-- 
2.5.0

[PATCH 1/2] zsmalloc: use page->private instead of page->first_page

[Kirill A. Shutemov]

We are going to rework how compound_head() work. It will not use
page->first_page as we have it now.

The only other user of page->fisrt_page beyond compound pages is
zsmalloc.

Let's use page->private instead of page->first_page here. It occupies
the same storage space.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 mm/zsmalloc.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 0a7f81aa2249..a85754e69879 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -16,7 +16,7 @@
  * struct page(s) to form a zspage.
  *
  * Usage of struct page fields:
- *	page->first_page: points to the first component (0-order) page
+ *	page->private: points to the first component (0-order) page
  *	page->index (union with page->freelist): offset of the first object
  *		starting in this page. For the first page, this is
  *		always 0, so we use this field (aka freelist) to point
@@ -26,8 +26,7 @@
  *
  *	For _first_ page only:
  *
- *	page->private (union with page->first_page): refers to the
- *		component page after the first page
+ *	page->private: refers to the component page after the first page
  *		If the page is first_page for huge object, it stores handle.
  *		Look at size_class->huge.
  *	page->freelist: points to the first free object in zspage.
@@ -770,7 +769,7 @@ static struct page *get_first_page(struct page *page)
 	if (is_first_page(page))
 		return page;
 	else
-		return page->first_page;
+		return (struct page *)page_private(page);
 }
 
 static struct page *get_next_page(struct page *page)
@@ -955,7 +954,7 @@ static struct page *alloc_zspage(struct size_class *class, gfp_t flags)
 	 * Allocate individual pages and link them together as:
 	 * 1. first page->private = first sub-page
 	 * 2. all sub-pages are linked together using page->lru
-	 * 3. each sub-page is linked to the first page using page->first_page
+	 * 3. each sub-page is linked to the first page using page->private
 	 *
 	 * For each size class, First/Head pages are linked together using
 	 * page->lru. Also, we set PG_private to identify the first page
@@ -980,7 +979,7 @@ static struct page *alloc_zspage(struct size_class *class, gfp_t flags)
 		if (i == 1)
 			set_page_private(first_page, (unsigned long)page);
 		if (i >= 1)
-			page->first_page = first_page;
+			set_page_private(first_page, (unsigned long)first_page);
 		if (i >= 2)
 			list_add(&page->lru, &prev_page->lru);
 		if (i == class->pages_per_zspage - 1)	/* last page */
-- 
2.5.0

[PATCH 2/2] mm: make compound_head() robust

[Kirill A. Shutemov]

Hugh has pointed that compound_head() call can be unsafe in some
context. There's one example:

	CPU0					CPU1

isolate_migratepages_block()
  page_count()
    compound_head()
      !!PageTail() == true
					put_page()
					  tail->first_page = NULL
      head = tail->first_page
					alloc_pages(__GFP_COMP)
					   prep_compound_page()
					     tail->first_page = head
					     __SetPageTail(p);
      !!PageTail() == true
    <head == NULL dereferencing>

The race is pure theoretical. I don't it's possible to trigger it in
practice. But who knows.

We can fix the race by chaging how encode PageTail() and compound_head()
within struct page to be able to update them in one shot.

The patch introduces page->compound_head into union with
page->mem_cgroup.

Set bit 0 of page->compound_head means that the page is tail. If the bit
set, rest of the page->compound_head is pointer to head page. Otherwise,
the field is NULL or pointer to memory cgroup.

page->mem_cgroup currenly only used for small or head pages, so there
shouldn't be any conflicts.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
---
 Documentation/vm/split_page_table_lock |  4 +-
 arch/xtensa/configs/iss_defconfig      |  1 -
 include/linux/mm.h                     | 53 ++--------------------
 include/linux/mm_types.h               | 15 ++++---
 include/linux/page-flags.h             | 80 ++++++++--------------------------
 mm/Kconfig                             | 12 -----
 mm/debug.c                             |  7 ---
 mm/hugetlb.c                           |  8 +---
 mm/internal.h                          |  4 +-
 mm/memory-failure.c                    |  7 ---
 mm/page_alloc.c                        | 36 +++++++--------
 mm/swap.c                              |  4 +-
 12 files changed, 56 insertions(+), 175 deletions(-)

diff --git a/Documentation/vm/split_page_table_lock b/Documentation/vm/split_page_table_lock
index 6dea4fd5c961..62842a857dab 100644
--- a/Documentation/vm/split_page_table_lock
+++ b/Documentation/vm/split_page_table_lock
@@ -54,8 +54,8 @@ everything required is done by pgtable_page_ctor() and pgtable_page_dtor(),
 which must be called on PTE table allocation / freeing.
 
 Make sure the architecture doesn't use slab allocator for page table
-allocation: slab uses page->slab_cache and page->first_page for its pages.
-These fields share storage with page->ptl.
+allocation: slab uses page->slab_cache for its pages.
+This field shares storage with page->ptl.
 
 PMD split lock only makes sense if you have more than two page table
 levels.
diff --git a/arch/xtensa/configs/iss_defconfig b/arch/xtensa/configs/iss_defconfig
index e4d193e7a300..5c7c385f21c4 100644
--- a/arch/xtensa/configs/iss_defconfig
+++ b/arch/xtensa/configs/iss_defconfig
@@ -169,7 +169,6 @@ CONFIG_FLATMEM_MANUAL=y
 # CONFIG_SPARSEMEM_MANUAL is not set
 CONFIG_FLATMEM=y
 CONFIG_FLAT_NODE_MEM_MAP=y
-CONFIG_PAGEFLAGS_EXTENDED=y
 CONFIG_SPLIT_PTLOCK_CPUS=4
 # CONFIG_PHYS_ADDR_T_64BIT is not set
 CONFIG_ZONE_DMA_FLAG=1
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 2e872f92dbac..ce135e72e535 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -437,46 +437,6 @@ static inline void compound_unlock_irqrestore(struct page *page,
 #endif
 }
 
-static inline struct page *compound_head_by_tail(struct page *tail)
-{
-	struct page *head = tail->first_page;
-
-	/*
-	 * page->first_page may be a dangling pointer to an old
-	 * compound page, so recheck that it is still a tail
-	 * page before returning.
-	 */
-	smp_rmb();
-	if (likely(PageTail(tail)))
-		return head;
-	return tail;
-}
-
-/*
- * Since either compound page could be dismantled asynchronously in THP
- * or we access asynchronously arbitrary positioned struct page, there
- * would be tail flag race. To handle this race, we should call
- * smp_rmb() before checking tail flag. compound_head_by_tail() did it.
- */
-static inline struct page *compound_head(struct page *page)
-{
-	if (unlikely(PageTail(page)))
-		return compound_head_by_tail(page);
-	return page;
-}
-
-/*
- * If we access compound page synchronously such as access to
- * allocated page, there is no need to handle tail flag race, so we can
- * check tail flag directly without any synchronization primitive.
- */
-static inline struct page *compound_head_fast(struct page *page)
-{
-	if (unlikely(PageTail(page)))
-		return page->first_page;
-	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
@@ -525,7 +485,7 @@ static inline void get_huge_page_tail(struct page *page)
 	VM_BUG_ON_PAGE(!PageTail(page), page);
 	VM_BUG_ON_PAGE(page_mapcount(page) < 0, page);
 	VM_BUG_ON_PAGE(atomic_read(&page->_count) != 0, page);
-	if (compound_tail_refcounted(page->first_page))
+	if (compound_tail_refcounted(compound_head(page)))
 		atomic_inc(&page->_mapcount);
 }
 
@@ -548,13 +508,7 @@ static inline struct page *virt_to_head_page(const void *x)
 {
 	struct page *page = virt_to_page(x);
 
-	/*
-	 * We don't need to worry about synchronization of tail flag
-	 * when we call virt_to_head_page() since it is only called for
-	 * already allocated page and this page won't be freed until
-	 * this virt_to_head_page() is finished. So use _fast variant.
-	 */
-	return compound_head_fast(page);
+	return compound_head(page);
 }
 
 /*
@@ -1482,8 +1436,7 @@ static inline bool ptlock_init(struct page *page)
 	 * with 0. Make sure nobody took it in use in between.
 	 *
 	 * It can happen if arch try to use slab for page table allocation:
-	 * slab code uses page->slab_cache and page->first_page (for tail
-	 * pages), which share storage with page->ptl.
+	 * slab code uses page->slab_cache, which share storage with page->ptl.
 	 */
 	VM_BUG_ON_PAGE(*(unsigned long *)&page->ptl, page);
 	if (!ptlock_alloc(page))
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 0038ac7466fd..e0c4c0a8ec3d 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -155,7 +155,7 @@ struct page {
 #endif
 	};
 
-	/* Remainder is not double word aligned */
+	/* Fourth double word block */
 	union {
 		unsigned long private;		/* Mapping-private opaque data:
 					 	 * usually used for buffer_heads
@@ -172,13 +172,17 @@ struct page {
 #endif
 #endif
 		struct kmem_cache *slab_cache;	/* SL[AU]B: Pointer to slab */
-		struct page *first_page;	/* Compound tail pages */
 	};
 
-#ifdef CONFIG_MEMCG
-	struct mem_cgroup *mem_cgroup;
-#endif
+	union {
+		/* Bit zero of the word encode PageTail() */
+		struct mem_cgroup *mem_cgroup;	/* If bit zero is clear */
+		unsigned long compound_head;	/* If bit zero is set */
+	};
 
+	/* Remainder is not double word aligned */
+
+#if defined(WANT_PAGE_VIRTUAL)
 	/*
 	 * On machines where all RAM is mapped into kernel address space,
 	 * we can simply calculate the virtual address. On machines with
@@ -189,7 +193,6 @@ struct page {
 	 * Architectures with slow multiplication can define
 	 * WANT_PAGE_VIRTUAL in asm/page.h
 	 */
-#if defined(WANT_PAGE_VIRTUAL)
 	void *virtual;			/* Kernel virtual address (NULL if
 					   not kmapped, ie. highmem) */
 #endif /* WANT_PAGE_VIRTUAL */
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 41c93844fb1d..9b865158e452 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -86,12 +86,7 @@ enum pageflags {
 	PG_private,		/* If pagecache, has fs-private data */
 	PG_private_2,		/* If pagecache, has fs aux data */
 	PG_writeback,		/* Page is under writeback */
-#ifdef CONFIG_PAGEFLAGS_EXTENDED
 	PG_head,		/* A head page */
-	PG_tail,		/* A tail page */
-#else
-	PG_compound,		/* A compound page */
-#endif
 	PG_swapcache,		/* Swap page: swp_entry_t in private */
 	PG_mappedtodisk,	/* Has blocks allocated on-disk */
 	PG_reclaim,		/* To be reclaimed asap */
@@ -387,85 +382,46 @@ static inline void set_page_writeback_keepwrite(struct page *page)
 	test_set_page_writeback_keepwrite(page);
 }
 
-#ifdef CONFIG_PAGEFLAGS_EXTENDED
-/*
- * System with lots of page flags available. This allows separate
- * flags for PageHead() and PageTail() checks of compound pages so that bit
- * tests can be used in performance sensitive paths. PageCompound is
- * generally not used in hot code paths except arch/powerpc/mm/init_64.c
- * and arch/powerpc/kvm/book3s_64_vio_hv.c which use it to detect huge pages
- * and avoid handling those in real mode.
- */
 __PAGEFLAG(Head, head) CLEARPAGEFLAG(Head, head)
-__PAGEFLAG(Tail, tail)
 
-static inline int PageCompound(struct page *page)
-{
-	return page->flags & ((1L << PG_head) | (1L << PG_tail));
-
-}
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
-static inline void ClearPageCompound(struct page *page)
+static inline int PageTail(struct page *page)
 {
-	BUG_ON(!PageHead(page));
-	ClearPageHead(page);
+	return READ_ONCE(page->compound_head) & 1;
 }
-#endif
-
-#define PG_head_mask ((1L << PG_head))
 
-#else
-/*
- * Reduce page flag use as much as possible by overlapping
- * compound page flags with the flags used for page cache pages. Possible
- * because PageCompound is always set for compound pages and not for
- * pages on the LRU and/or pagecache.
- */
-TESTPAGEFLAG(Compound, compound)
-__SETPAGEFLAG(Head, compound)  __CLEARPAGEFLAG(Head, compound)
-
-/*
- * PG_reclaim is used in combination with PG_compound to mark the
- * head and tail of a compound page. This saves one page flag
- * but makes it impossible to use compound pages for the page cache.
- * The PG_reclaim bit would have to be used for reclaim or readahead
- * if compound pages enter the page cache.
- *
- * PG_compound & PG_reclaim	=> Tail page
- * PG_compound & ~PG_reclaim	=> Head page
- */
-#define PG_head_mask ((1L << PG_compound))
-#define PG_head_tail_mask ((1L << PG_compound) | (1L << PG_reclaim))
-
-static inline int PageHead(struct page *page)
+static inline void set_compound_head(struct page *page, struct page *head)
 {
-	return ((page->flags & PG_head_tail_mask) == PG_head_mask);
+	WRITE_ONCE(page->compound_head, (unsigned long)head + 1);
 }
 
-static inline int PageTail(struct page *page)
+static inline void clear_compound_head(struct page *page)
 {
-	return ((page->flags & PG_head_tail_mask) == PG_head_tail_mask);
+	WRITE_ONCE(page->compound_head, 0);
 }
 
-static inline void __SetPageTail(struct page *page)
+static inline struct page *compound_head(struct page *page)
 {
-	page->flags |= PG_head_tail_mask;
+	unsigned long head = READ_ONCE(page->compound_head);
+
+	if (unlikely(head & 1))
+		return (struct page *) (head - 1);
+	return page;
 }
 
-static inline void __ClearPageTail(struct page *page)
+static inline int PageCompound(struct page *page)
 {
-	page->flags &= ~PG_head_tail_mask;
-}
+	return PageHead(page) || PageTail(page);
 
+}
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 static inline void ClearPageCompound(struct page *page)
 {
-	BUG_ON((page->flags & PG_head_tail_mask) != (1 << PG_compound));
-	clear_bit(PG_compound, &page->flags);
+	BUG_ON(!PageHead(page));
+	ClearPageHead(page);
 }
 #endif
 
-#endif /* !PAGEFLAGS_EXTENDED */
+#define PG_head_mask ((1L << PG_head))
 
 #ifdef CONFIG_HUGETLB_PAGE
 int PageHuge(struct page *page);
diff --git a/mm/Kconfig b/mm/Kconfig
index e79de2bd12cd..454579d31081 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -200,18 +200,6 @@ config MEMORY_HOTREMOVE
 	depends on MEMORY_HOTPLUG && ARCH_ENABLE_MEMORY_HOTREMOVE
 	depends on MIGRATION
 
-#
-# If we have space for more page flags then we can enable additional
-# optimizations and functionality.
-#
-# Regular Sparsemem takes page flag bits for the sectionid if it does not
-# use a virtual memmap. Disable extended page flags for 32 bit platforms
-# that require the use of a sectionid in the page flags.
-#
-config PAGEFLAGS_EXTENDED
-	def_bool y
-	depends on 64BIT || SPARSEMEM_VMEMMAP || !SPARSEMEM
-
 # Heavily threaded applications may benefit from splitting the mm-wide
 # page_table_lock, so that faults on different parts of the user address
 # space can be handled with less contention: split it at this NR_CPUS.
diff --git a/mm/debug.c b/mm/debug.c
index 76089ddf99ea..721502d40c43 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -25,12 +25,7 @@ static const struct trace_print_flags pageflag_names[] = {
 	{1UL << PG_private,		"private"	},
 	{1UL << PG_private_2,		"private_2"	},
 	{1UL << PG_writeback,		"writeback"	},
-#ifdef CONFIG_PAGEFLAGS_EXTENDED
 	{1UL << PG_head,		"head"		},
-	{1UL << PG_tail,		"tail"		},
-#else
-	{1UL << PG_compound,		"compound"	},
-#endif
 	{1UL << PG_swapcache,		"swapcache"	},
 	{1UL << PG_mappedtodisk,	"mappedtodisk"	},
 	{1UL << PG_reclaim,		"reclaim"	},
@@ -95,10 +90,8 @@ void dump_page_badflags(struct page *page, const char *reason,
 		dump_flags(page->flags & badflags,
 				pageflag_names, ARRAY_SIZE(pageflag_names));
 	}
-#ifdef CONFIG_MEMCG
 	if (page->mem_cgroup)
 		pr_alert("page->mem_cgroup:%p\n", page->mem_cgroup);
-#endif
 }
 
 void dump_page(struct page *page, const char *reason)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index a8c3087089d8..82b99d994c69 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -824,9 +824,8 @@ static void destroy_compound_gigantic_page(struct page *page,
 	struct page *p = page + 1;
 
 	for (i = 1; i < nr_pages; i++, p = mem_map_next(p, page, i)) {
-		__ClearPageTail(p);
+		clear_compound_head(p);
 		set_page_refcounted(p);
-		p->first_page = NULL;
 	}
 
 	set_compound_order(page, 0);
@@ -1099,10 +1098,7 @@ static void prep_compound_gigantic_page(struct page *page, unsigned long order)
 		 */
 		__ClearPageReserved(p);
 		set_page_count(p, 0);
-		p->first_page = page;
-		/* Make sure p->first_page is always valid for PageTail() */
-		smp_wmb();
-		__SetPageTail(p);
+		set_compound_head(p, page);
 	}
 }
 
diff --git a/mm/internal.h b/mm/internal.h
index 36b23f1e2ca6..89e21a07080a 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -61,9 +61,9 @@ static inline void __get_page_tail_foll(struct page *page,
 	 * speculative page access (like in
 	 * page_cache_get_speculative()) on tail pages.
 	 */
-	VM_BUG_ON_PAGE(atomic_read(&page->first_page->_count) <= 0, page);
+	VM_BUG_ON_PAGE(atomic_read(&compound_head(page)->_count) <= 0, page);
 	if (get_page_head)
-		atomic_inc(&page->first_page->_count);
+		atomic_inc(&compound_head(page)->_count);
 	get_huge_page_tail(page);
 }
 
diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index ea5a93659488..f4e54be1e92a 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -787,8 +787,6 @@ static int me_huge_page(struct page *p, unsigned long pfn)
 #define lru		(1UL << PG_lru)
 #define swapbacked	(1UL << PG_swapbacked)
 #define head		(1UL << PG_head)
-#define tail		(1UL << PG_tail)
-#define compound	(1UL << PG_compound)
 #define slab		(1UL << PG_slab)
 #define reserved	(1UL << PG_reserved)
 
@@ -811,12 +809,7 @@ static struct page_state {
 	 */
 	{ slab,		slab,		MF_MSG_SLAB,	me_kernel },
 
-#ifdef CONFIG_PAGEFLAGS_EXTENDED
 	{ head,		head,		MF_MSG_HUGE,		me_huge_page },
-	{ tail,		tail,		MF_MSG_HUGE,		me_huge_page },
-#else
-	{ compound,	compound,	MF_MSG_HUGE,		me_huge_page },
-#endif
 
 	{ sc|dirty,	sc|dirty,	MF_MSG_DIRTY_SWAPCACHE,	me_swapcache_dirty },
 	{ sc|dirty,	sc,		MF_MSG_CLEAN_SWAPCACHE,	me_swapcache_clean },
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index beda41710802..a5beebc033a2 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -419,7 +419,7 @@ out:
  *
  * The remaining PAGE_SIZE pages are called "tail pages".
  *
- * All pages have PG_compound set.  All tail pages have their ->first_page
+ * All pages have PG_compound set.  All tail pages have their compound_head()
  * pointing at the head page.
  *
  * The first tail page's ->lru.next holds the address of the compound page's
@@ -443,10 +443,7 @@ void prep_compound_page(struct page *page, unsigned long order)
 	for (i = 1; i < nr_pages; i++) {
 		struct page *p = page + i;
 		set_page_count(p, 0);
-		p->first_page = page;
-		/* Make sure p->first_page is always valid for PageTail() */
-		smp_wmb();
-		__SetPageTail(p);
+		set_compound_head(p, page);
 	}
 }
 
@@ -722,10 +719,8 @@ static inline int free_pages_check(struct page *page)
 		bad_reason = "PAGE_FLAGS_CHECK_AT_FREE flag(s) set";
 		bad_flags = PAGE_FLAGS_CHECK_AT_FREE;
 	}
-#ifdef CONFIG_MEMCG
 	if (unlikely(page->mem_cgroup))
-		bad_reason = "page still charged to cgroup";
-#endif
+		bad_reason = "non-NULL mem_cgroup";
 	if (unlikely(bad_reason)) {
 		bad_page(page, bad_reason, bad_flags);
 		return 1;
@@ -821,17 +816,24 @@ static void free_one_page(struct zone *zone,
 
 static int free_tail_pages_check(struct page *head_page, struct page *page)
 {
-	if (!IS_ENABLED(CONFIG_DEBUG_VM))
-		return 0;
+	int ret = 1;
+
+	if (!IS_ENABLED(CONFIG_DEBUG_VM)) {
+		ret = 0;
+		goto out;
+	}
 	if (unlikely(!PageTail(page))) {
 		bad_page(page, "PageTail not set", 0);
-		return 1;
+		goto out;
 	}
-	if (unlikely(page->first_page != head_page)) {
-		bad_page(page, "first_page not consistent", 0);
-		return 1;
+	if (unlikely(compound_head(page) != head_page)) {
+		bad_page(page, "compound_head not consistent", 0);
+		goto out;
 	}
-	return 0;
+	ret = 0;
+out:
+	clear_compound_head(page);
+	return ret;
 }
 
 static void __meminit __init_single_page(struct page *page, unsigned long pfn,
@@ -1304,10 +1306,8 @@ static inline int check_new_page(struct page *page)
 		bad_reason = "PAGE_FLAGS_CHECK_AT_PREP flag set";
 		bad_flags = PAGE_FLAGS_CHECK_AT_PREP;
 	}
-#ifdef CONFIG_MEMCG
 	if (unlikely(page->mem_cgroup))
-		bad_reason = "page still charged to cgroup";
-#endif
+		bad_reason = "non-NULL mem_cgroup";
 	if (unlikely(bad_reason)) {
 		bad_page(page, bad_reason, bad_flags);
 		return 1;
diff --git a/mm/swap.c b/mm/swap.c
index a3a0a2f1f7c3..faa9e1687dea 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -200,7 +200,7 @@ out_put_single:
 				__put_single_page(page);
 			return;
 		}
-		VM_BUG_ON_PAGE(page_head != page->first_page, page);
+		VM_BUG_ON_PAGE(page_head != compound_head(page), page);
 		/*
 		 * We can release the refcount taken by
 		 * get_page_unless_zero() now that
@@ -261,7 +261,7 @@ static void put_compound_page(struct page *page)
 	 *  Case 3 is possible, as we may race with
 	 *  __split_huge_page_refcount tearing down a THP page.
 	 */
-	page_head = compound_head_by_tail(page);
+	page_head = compound_head(page);
 	if (!__compound_tail_refcounted(page_head))
 		put_unrefcounted_compound_page(page_head, page);
 	else
-- 
2.5.0

Re: [PATCH 2/2] mm: make compound_head() robust

[Hugh Dickins]

On Thu, 13 Aug 2015, Kirill A. Shutemov wrote:

> Hugh has pointed that compound_head() call can be unsafe in some
> context. There's one example:
> 
> 	CPU0					CPU1
> 
> isolate_migratepages_block()
>   page_count()
>     compound_head()
>       !!PageTail() == true
> 					put_page()
> 					  tail->first_page = NULL
>       head = tail->first_page
> 					alloc_pages(__GFP_COMP)
> 					   prep_compound_page()
> 					     tail->first_page = head
> 					     __SetPageTail(p);
>       !!PageTail() == true
>     <head == NULL dereferencing>
> 
> The race is pure theoretical. I don't it's possible to trigger it in
> practice. But who knows.
> 
> We can fix the race by chaging how encode PageTail() and compound_head()
> within struct page to be able to update them in one shot.
> 
> The patch introduces page->compound_head into union with
> page->mem_cgroup.
> 
> Set bit 0 of page->compound_head means that the page is tail. If the bit
> set, rest of the page->compound_head is pointer to head page. Otherwise,
> the field is NULL or pointer to memory cgroup.
> 
> page->mem_cgroup currenly only used for small or head pages, so there
> shouldn't be any conflicts.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> ---
>  Documentation/vm/split_page_table_lock |  4 +-
>  arch/xtensa/configs/iss_defconfig      |  1 -
>  include/linux/mm.h                     | 53 ++--------------------
>  include/linux/mm_types.h               | 15 ++++---
>  include/linux/page-flags.h             | 80 ++++++++--------------------------
>  mm/Kconfig                             | 12 -----
>  mm/debug.c                             |  7 ---
>  mm/hugetlb.c                           |  8 +---
>  mm/internal.h                          |  4 +-
>  mm/memory-failure.c                    |  7 ---
>  mm/page_alloc.c                        | 36 +++++++--------
>  mm/swap.c                              |  4 +-
>  12 files changed, 56 insertions(+), 175 deletions(-)

Mostly I like this: especially those deletions,
and removing the unloved CONFIG_PAGEFLAGS_EXTENDED.

But I do disagree with:

> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 0038ac7466fd..e0c4c0a8ec3d 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -172,13 +172,17 @@ struct page {
>  #endif
>  #endif
>  		struct kmem_cache *slab_cache;	/* SL[AU]B: Pointer to slab */
> -		struct page *first_page;	/* Compound tail pages */
>  	};
>  
> -#ifdef CONFIG_MEMCG
> -	struct mem_cgroup *mem_cgroup;
> -#endif
> +	union {
> +		/* Bit zero of the word encode PageTail() */
> +		struct mem_cgroup *mem_cgroup;	/* If bit zero is clear */
> +		unsigned long compound_head;	/* If bit zero is set */
> +	};

On 32-bit, I think the addition of that mem_cgroup pointer enlarged
struct page from 32 bytes to 36 (SLAB or SLOB) or 40 (SLUB) bytes.
I can well imagine people wanting to cut that bloat by turning off
CONFIG_MEMCG, but now you would be thwarting them.  (I can also
imagine memcg people might want to add flag bits of their own to it.)

My own preference (Andrew might disagree) would be to give up on
compound_page_dtor *compound_dtor: in all the years it's been there,
it has only been assigned two possibilities, and I think you'd do
well to hard code those in the one or two places they're needed -
moving PageHuge to be a second bit alongside your PageTail (but of
course it could only be set in the first tail page, not the head).

But as far as fixing the isolate_migratepages_block() bug you
discovered, I think your original atomic_read(&page->_count) fix
was good enough, or a page_count_raw(page) if Andrew prefers -
though I'm not so keen on dressing these things up myself, such
raw scans are exceptional and face very special problems, I like
to see exactly what's going on in them.

And even when a race between PageTail and first_page is resolved
by your READ_ONCE, it still leaves races of whether the head page
still agrees with the tail.  Safer races now because first_page is
reliably a recent first_page pointer: which might be all that's needed
to make your refcounting patches' wider use of compound_head() safe -
this patch does look like a good step for those.

Hugh