Re: Process-shared futexes on hugepages puts the kernel in aninfinite loop in 2.6.32.11; is this fixed now?

[Peter Zijlstra]

On Mon, 2010-04-19 at 16:32 +0100, Mel Gorman wrote:
> Fix infinite loop in get_futex_key when backed by huge pages
> 
> If a futex key happens to be located within a huge page mapped MAP_PRIVATE,
> get_futex_key() can go into an infinite loop waiting for a page->mapping
> that will never exist. This was reported and documented in an external
> bugzilla at
> 
> https://bugzilla.redhat.com/show_bug.cgi?id=552257
> 
> This patch makes page->mapping a poisoned value that includes PAGE_MAPPING_ANON
> mapped MAP_PRIVATE.  This is enough for futex to continue but because
> of PAGE_MAPPING_ANON, the poisoned value is not dereferenced or used by
> futex. No other part of the VM should be dereferencing the page->mapping of
> a hugetlbfs page as its page cache is not on the LRU.
> 
> This patch fixes the problem with the test case described in the bugzilla.
> 
> Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
> ---
>  include/linux/poison.h |   10 ++++++++++
>  mm/hugetlb.c           |    6 +++++-
>  2 files changed, 15 insertions(+), 1 deletions(-)
> 
> diff --git a/include/linux/poison.h b/include/linux/poison.h
> index 2110a81..0f7b5ac 100644
> --- a/include/linux/poison.h
> +++ b/include/linux/poison.h
> @@ -48,6 +48,16 @@
>  #define POISON_FREE    0x6b    /* for use-after-free poisoning */
>  #define        POISON_END      0xa5    /* end-byte of poisoning */
>  
> +/********** mm/hugetlb.c **********/
> +/*
> + * Private mappings of hugetlb pages use this poisoned value for
> + * page->mapping. The core VM should not be doing anything with this mapping
> + * but futex requires the existance of some page->mapping value even if it
> + * is unused. If the core VM does deference the mapping, it'll look like a
> + * suspiciously high null-pointer offset starting from 0x2e5
> + */
> +#define HUGETLB_PRIVATE_MAPPING        (0x2e4 | PAGE_MAPPING_ANON)

Wouldn't a longer poison be more recognisable? Also, shouldn't this use
POISON_POINTER_DELTA?

Something like:

#define HUGETBL_POISON	((void *) 0x00300300 + POISON_POINTER_DELTA)

0x2e5 isn't that high, I've had actual derefs in that range.

> +
>  /********** arch/$ARCH/mm/init.c **********/
>  #define POISON_FREE_INITMEM    0xcc
>  
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 6034dc9..487e3c2 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -546,6 +546,7 @@ static void free_huge_page(struct page *page)
>  
>         mapping = (struct address_space *) page_private(page);
>         set_page_private(page, 0);
> +       page->mapping = NULL;
>         BUG_ON(page_count(page));
>         INIT_LIST_HEAD(&page->lru);
>  
> @@ -2447,8 +2448,11 @@ retry:
>                         spin_lock(&inode->i_lock);
>                         inode->i_blocks += blocks_per_huge_page(h);
>                         spin_unlock(&inode->i_lock);
> -               } else
> +               } else {
>                         lock_page(page);
> +                       page->mapping = (struct address_space *)
> +                                                       HUGETLB_PRIVATE_MAPPING;
> +               }
>         }
>  
>         /*
> 
> 

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