Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753122AbbHSJ1M (ORCPT ); Wed, 19 Aug 2015 05:27:12 -0400 Received: from mga14.intel.com ([192.55.52.115]:4080 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750999AbbHSJ1G (ORCPT ); Wed, 19 Aug 2015 05:27:06 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.15,709,1432623600"; d="scan'208,217";a="751414619" From: "Kirill A. Shutemov" To: Andrew Morton , Hugh Dickins Cc: Andrea Arcangeli , Dave Hansen , Vlastimil Babka , Johannes Weiner , Michal Hocko , David Rientjes , linux-kernel@vger.kernel.org, linux-mm@kvack.org, "Kirill A. Shutemov" Subject: [PATCHv3 4/5] mm: make compound_head() robust Date: Wed, 19 Aug 2015 12:21:45 +0300 Message-Id: <1439976106-137226-5-git-send-email-kirill.shutemov@linux.intel.com> X-Mailer: git-send-email 2.5.0 In-Reply-To: <1439976106-137226-1-git-send-email-kirill.shutemov@linux.intel.com> References: <1439976106-137226-1-git-send-email-kirill.shutemov@linux.intel.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 18508 Lines: 549 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 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 Acked-by: Michal Hocko Cc: Hugh Dickins Cc: David Rientjes Cc: Vlastimil Babka --- Documentation/vm/split_page_table_lock | 4 +- arch/xtensa/configs/iss_defconfig | 1 - include/linux/mm.h | 53 ++-------------------- include/linux/mm_types.h | 9 +++- 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 | 38 ++++++++-------- mm/swap.c | 4 +- 13 files changed, 58 insertions(+), 170 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 0735bc0a351a..a4c4b7d07473 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); } /* @@ -1496,8 +1450,7 @@ 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..e324768b6cc7 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; @@ -174,7 +180,6 @@ 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 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 c6733cc3cbce..78859d47aaf4 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -424,15 +424,15 @@ out: /* * 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. */ @@ -452,10 +452,7 @@ 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); } } @@ -830,17 +827,24 @@ 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/