[PATCHv2 4/4] mm: make compound_head() robust

From: Kirill A. Shutemov
Date: Mon Aug 17 2015 - 11:10:02 EST

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 changing 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 third double word block in
front of compound_dtor and compound_order. That means it shares storage
space with:

- page->lru.next;
- page->next;
- page->rcu_head.next;
- page->pmd_huge_pte;

That's too long list to be absolutely sure, but looks like nobody uses
bit 0 of the word. It can be used to encode PageTail(). And if the bit
set, rest of the word is pointer to head page.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
Documentation/vm/split_page_table_lock | 4 +-
arch/xtensa/configs/iss_defconfig | 1 -
include/linux/mm.h | 53 ++--------------------
include/linux/mm_types.h | 8 +++-
include/linux/page-flags.h | 80 ++++++++--------------------------
mm/Kconfig | 12 -----
mm/debug.c | 5 ---
mm/huge_memory.c | 3 +-
mm/hugetlb.c | 8 +---
mm/internal.h | 4 +-
mm/memory-failure.c | 7 ---
mm/page_alloc.c | 28 +++++++-----
mm/swap.c | 4 +-
13 files changed, 53 insertions(+), 164 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 9c21bbb8875a..5090a0b9bb43 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);
}

/*
@@ -1494,8 +1448,7 @@ 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 63cdfe7ec336..ad47b61d96ce 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -120,7 +120,12 @@ struct page {
};
};

- /* Third double word block */
+ /*
+ * Third double word block
+ *
+ * WARNING: bit 0 of the first word encode PageTail and *must* be 0
+ * for non-tail pages.
+ */
union {
struct list_head lru; /* Pageout list, eg. active_list
* protected by zone->lru_lock !
@@ -143,6 +148,7 @@ struct page {
*/
/* First tail page of compound page */
struct {
+ unsigned long compound_head; /* If bit zero is set */
#ifdef CONFIG_64BIT
unsigned int compound_dtor;
unsigned int compound_order;
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..205e5ef957ab 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" },
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 097c7a4bfbd9..330377f83ac7 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1686,8 +1686,7 @@ static void __split_huge_page_refcount(struct page *page,
(1L << PG_unevictable)));
page_tail->flags |= (1L << PG_dirty);

- /* clear PageTail before overwriting first_page */
- smp_wmb();
+ clear_compound_head(page_tail);

/*
* __split_huge_page_splitting() already set the
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 8ea74caa1fa8..53c0709fd87b 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 1f4446a90cef..4d1a5de9653d 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 beab86e694b2..16c3f97a7d30 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -426,7 +426,7 @@ 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
@@ -450,10 +450,7 @@ 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);
}
}

@@ -828,17 +825,24 @@ 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,
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

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