On Mon 17-08-15 18:09:05, 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 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.
I didn't look too closely but the general idea makes sense to me and the
overal code simplification is sound. I will give it more detailed review
after I sort out other stuff.
>
> 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
--
Michal Hocko
SUSE Labs
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