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>
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.
# 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
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
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;
}
#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
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);
}
{
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);
}
/*
* 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))
};
};
- /* 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 !
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
};
#endif
#endif
struct kmem_cache *slab_cache; /* SL[AU]B: Pointer to slab */
- struct page *first_page; /* Compound tail pages */
};
#ifdef CONFIG_MEMCG
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 */
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);
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.
{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" },
(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);
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);
*/
__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);
}
}
/*
* 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));
* 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);
}
#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)
*/
{ 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 },
/*
* 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.
*/
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);
}
}
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,
struct page *page = pfn_to_page(start_pfn);
init_reserved_page(start_pfn);
+
+ /* Avoid false-positive PageTail() */
+ INIT_LIST_HEAD(&page->lru);
+
SetPageReserved(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
* 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
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