Re: [PATCHv2 4/4] mm: make compound_head() robust
From: Michal Hocko
Date: Tue Aug 18 2015 - 12:41:21 EST
On Tue 18-08-15 13:20:22, Michal Hocko wrote:
> 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.
AFICS page::first_page wasn't used outside of compound page logic so you
should remove it in this patch. The rest looks good to me.
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> > 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
--
Michal Hocko
SUSE Labs
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