diff options
| author | Kirill A. Shutemov | 2015-11-06 16:29:54 -0800 |
|---|---|---|
| committer | Linus Torvalds | 2015-11-06 17:50:42 -0800 |
| commit | 1d798ca3f16437c71ff63e36597ff07f9c12e4d6 (patch) | |
| tree | 4b70d32439fb18ef699175413e4b82c4af206f81 /include | |
| parent | f1e61557f0230d51a3df8d825f2c156e75563bff (diff) | |
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
<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>
Diffstat (limited to 'include')
| -rw-r--r-- | include/linux/hugetlb_cgroup.h | 4 | ||||
| -rw-r--r-- | include/linux/mm.h | 53 | ||||
| -rw-r--r-- | include/linux/mm_types.h | 22 | ||||
| -rw-r--r-- | include/linux/page-flags.h | 80 |
4 files changed, 41 insertions, 118 deletions
diff --git a/include/linux/hugetlb_cgroup.h b/include/linux/hugetlb_cgroup.h index 7edd30515298..24154c26d469 100644 --- a/include/linux/hugetlb_cgroup.h +++ b/include/linux/hugetlb_cgroup.h @@ -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; } diff --git a/include/linux/mm.h b/include/linux/mm.h index 6581c21320cb..9671b6f23eda 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -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)) diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index e334ef79cb43..bb91658c603f 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -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 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); |