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mm: make compound_head() robust

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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. Bit 0 encodes PageTail() and
the rest bits are pointer to head page if bit zero is set.

The patch moves page->pmd_huge_pte out of word, just in case if an
architecture defines pgtable_t into something what can have the bit 0
set.

hugetlb_cgroup uses page->lru.next in the second tail page to store
pointer struct hugetlb_cgroup. The patch switch it to use page->private
in the second tail page instead. The space is free since ->first_page is
removed from the union.

The patch also opens possibility to remove HUGETLB_CGROUP_MIN_ORDER
limitation, since there's now space in first tail page to store struct
hugetlb_cgroup pointer. But that's out of scope of the patch.

That means page->compound_head shares storage space with:

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

That's too long list to be absolutely sure, but looks like nobody uses
bit 0 of the word.

page->rcu_head.next guaranteed[1] to have bit 0 clean as long as we use
call_rcu(), call_rcu_bh(), call_rcu_sched(), or call_srcu(). But future
call_rcu_lazy() is not allowed as it makes use of the bit and we can
get false positive PageTail().

[1] http://lkml.kernel.org/g/20150827163634.GD4029@linux.vnet.ibm.com

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Kirill A. Shutemov 2015-11-06 16:29:54 -08:00 committed by Linus Torvalds
parent f1e61557f0
commit 1d798ca3f1
15 changed files with 89 additions and 182 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

4
Documentation/vm/split_page_table_lock
View file

@ -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.

1
arch/xtensa/configs/iss_defconfig
View file

@ -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

4
include/linux/hugetlb_cgroup.h
View file

@ -32,7 +32,7 @@ static inline struct hugetlb_cgroup *hugetlb_cgroup_from_page(struct page *page)
if (compound_order(page) < HUGETLB_CGROUP_MIN_ORDER)
return NULL;
return (struct hugetlb_cgroup *)page[2].lru.next;
return (struct hugetlb_cgroup *)page[2].private;
}
static inline
@ -42,7 +42,7 @@ int set_hugetlb_cgroup(struct page *page, struct hugetlb_cgroup *h_cg)
if (compound_order(page) < HUGETLB_CGROUP_MIN_ORDER)
return -1;
page[2].lru.next = (void *)h_cg;
page[2].private = (unsigned long)h_cg;
return 0;
}

53
include/linux/mm.h
View file

@ -430,46 +430,6 @@ 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
@ -518,7 +478,7 @@ 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);
}
@ -541,13 +501,7 @@ 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);
}
/*
@ -1586,8 +1540,7 @@ 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))

22
include/linux/mm_types.h
View file

@ -111,7 +111,13 @@ struct page {
};
};
/* Third double word block */
/*
* Third double word block
*
* WARNING: bit 0 of the first word encode PageTail(). That means
* the rest users of the storage space MUST NOT use the bit to
* avoid collision and false-positive PageTail().
*/
union {
struct list_head lru; /* Pageout list, eg. active_list
* protected by zone->lru_lock !
@ -132,14 +138,23 @@ struct page {
struct rcu_head rcu_head; /* Used by SLAB
* when destroying via RCU
*/
/* First tail page of compound page */
/* Tail pages of compound page */
struct {
unsigned long compound_head; /* If bit zero is set */
/* First tail page only */
unsigned short int compound_dtor;
unsigned short int compound_order;
};
#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && USE_SPLIT_PMD_PTLOCKS
pgtable_t pmd_huge_pte; /* protected by page->ptl */
struct {
unsigned long __pad; /* do not overlay pmd_huge_pte
* with compound_head to avoid
* possible bit 0 collision.
*/
pgtable_t pmd_huge_pte; /* protected by page->ptl */
};
#endif
};
@ -160,7 +175,6 @@ 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

94
include/linux/page-flags.h
View file

@ -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 */
@ -398,21 +393,35 @@ 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 PageTail(struct page *page)
{
return READ_ONCE(page->compound_head) & 1;
}
static inline void set_compound_head(struct page *page, struct page *head)
{
WRITE_ONCE(page->compound_head, (unsigned long)head + 1);
}
static inline void clear_compound_head(struct page *page)
{
WRITE_ONCE(page->compound_head, 0);
}
static inline struct page *compound_head(struct page *page)
{
unsigned long head = READ_ONCE(page->compound_head);
if (unlikely(head & 1))
return (struct page *) (head - 1);
return page;
}
static inline int PageCompound(struct page *page)
{
return page->flags & ((1L << PG_head) | (1L << PG_tail));
return PageHead(page) || PageTail(page);
}
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
@ -425,59 +434,6 @@ static inline void ClearPageCompound(struct page *page)
#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)
{
return ((page->flags & PG_head_tail_mask) == PG_head_mask);
}
static inline int PageTail(struct page *page)
{
return ((page->flags & PG_head_tail_mask) == PG_head_tail_mask);
}
static inline void __SetPageTail(struct page *page)
{
page->flags |= PG_head_tail_mask;
}
static inline void __ClearPageTail(struct page *page)
{
page->flags &= ~PG_head_tail_mask;
}
#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);
}
#endif
#endif /* !PAGEFLAGS_EXTENDED */
#ifdef CONFIG_HUGETLB_PAGE
int PageHuge(struct page *page);
int PageHeadHuge(struct page *page);

12
mm/Kconfig
View file

@ -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.

5
mm/debug.c
View file

@ -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" },

3
mm/huge_memory.c
View file

@ -1755,8 +1755,7 @@ 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);
if (page_is_young(page))
set_page_young(page_tail);

8
mm/hugetlb.c
View file

@ -1001,9 +1001,8 @@ 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);
@ -1276,10 +1275,7 @@ 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);
}
}

2
mm/hugetlb_cgroup.c
View file

@ -385,7 +385,7 @@ void __init hugetlb_cgroup_file_init(void)
/*
* Add cgroup control files only if the huge page consists
* of more than two normal pages. This is because we use
* page[2].lru.next for storing cgroup details.
* page[2].private for storing cgroup details.
*/
if (huge_page_order(h) >= HUGETLB_CGROUP_MIN_ORDER)
__hugetlb_cgroup_file_init(hstate_index(h));

4
mm/internal.h
View file

@ -80,9 +80,9 @@ 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);
}

7
mm/memory-failure.c
View file

@ -776,8 +776,6 @@ 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)
@ -800,12 +798,7 @@ 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 },

48
mm/page_alloc.c
View file

@ -445,15 +445,15 @@ static void bad_page(struct page *page, const char *reason,
/*
* Higher-order pages are called "compound pages". They are structured thusly:
*
* The first PAGE_SIZE page is called the "head page".
* The first PAGE_SIZE page is called the "head page" and have PG_head set.
*
* The remaining PAGE_SIZE pages are called "tail pages".
* The remaining PAGE_SIZE pages are called "tail pages". PageTail() is encoded
* in bit 0 of page->compound_head. The rest of bits is pointer to head page.
*
* All pages have PG_compound set. All tail pages have their ->first_page
* pointing at the head page.
* The first tail page's ->compound_dtor holds the offset in array of compound
* page destructors. See compound_page_dtors.
*
* The first tail page's ->lru.next holds the address of the compound page's
* put_page() function. Its ->lru.prev holds the order of allocation.
* The first tail page's ->compound_order holds the order of allocation.
* This usage means that zero-order pages may not be compound.
*/
@ -473,10 +473,7 @@ 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);
}
}
@ -854,17 +851,30 @@ 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;
/*
* We rely page->lru.next never has bit 0 set, unless the page
* is PageTail(). Let's make sure that's true even for poisoned ->lru.
*/
BUILD_BUG_ON((unsigned long)LIST_POISON1 & 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,
@ -931,6 +941,10 @@ void __meminit reserve_bootmem_region(unsigned long start, unsigned long end)
struct page *page = pfn_to_page(start_pfn);
init_reserved_page(start_pfn);
/* Avoid false-positive PageTail() */
INIT_LIST_HEAD(&page->lru);
SetPageReserved(page);
}
}

4
mm/swap.c
View file

@ -201,7 +201,7 @@ void put_refcounted_compound_page(struct page *page_head, struct page *page)
__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
@ -262,7 +262,7 @@ 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