From 1d798ca3f16437c71ff63e36597ff07f9c12e4d6 Mon Sep 17 00:00:00 2001 From: "Kirill A. Shutemov" Date: Fri, 6 Nov 2015 16:29:54 -0800 Subject: mm: make compound_head() robust 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. 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 Acked-by: Michal Hocko Reviewed-by: Andrea Arcangeli Cc: Hugh Dickins Cc: David Rientjes Cc: Vlastimil Babka Acked-by: Paul E. McKenney Cc: Aneesh Kumar K.V Cc: Andi Kleen Cc: Christoph Lameter Cc: Joonsoo Kim Cc: Sergey Senozhatsky Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/linux/page-flags.h | 80 +++++++++++----------------------------------- 1 file changed, 18 insertions(+), 62 deletions(-) (limited to 'include/linux/page-flags.h') diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h index a525e5067484..bb53c7b86315 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 */ @@ -398,85 +393,46 @@ 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); -- cgit