diff options
| author | Kirill A. Shutemov <kirill.shutemov@linux.intel.com> | 2015-11-07 03:29:54 +0300 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2015-11-07 04:50:42 +0300 |
| commit | 1d798ca3f16437c71ff63e36597ff07f9c12e4d6 (patch) | |
| tree | 4b70d32439fb18ef699175413e4b82c4af206f81 /include/linux/page-flags.h | |
| parent | f1e61557f0230d51a3df8d825f2c156e75563bff (diff) | |
| download | linux-1d798ca3f16437c71ff63e36597ff07f9c12e4d6.tar.xz | |
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/linux/page-flags.h')
| -rw-r--r-- | include/linux/page-flags.h | 80 |
1 files changed, 18 insertions, 62 deletions
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); |