Re: [PATCH] mm/hugetlb: clear compound_mapcount when freeing gigantic pages

[Mike Kravetz]

On 06/22/2016 09:25 AM, Gerald Schaefer wrote:
> While working on s390 support for gigantic hugepages I ran into the following
> "Bad page state" warning when freeing gigantic pages:
> 
> BUG: Bad page state in process bash  pfn:580001
> page:000003d116000040 count:0 mapcount:0 mapping:ffffffff00000000 index:0x0
> flags: 0x7fffc0000000000()
> page dumped because: non-NULL mapping
> 
> This is because page->compound_mapcount, which is part of a union with
> page->mapping, is initialized with -1 in prep_compound_gigantic_page(), and
> not cleared again during destroy_compound_gigantic_page(). Fix this by
> clearing the compound_mapcount in destroy_compound_gigantic_page() before
> clearing compound_head.
> 
> Interestingly enough, the warning will not show up on x86_64, although this
> should not be architecture specific. Apparently there is an endianness issue,
> combined with the fact that the union contains both a 64 bit ->mapping
> pointer and a 32 bit atomic_t ->compound_mapcount as members. The resulting
> bogus page->mapping on x86_64 therefore contains 00000000ffffffff instead
> of ffffffff00000000 on s390, which will falsely trigger the PageAnon() check
> in free_pages_prepare() because page->mapping & PAGE_MAPPING_ANON is true
> on little-endian architectures like x86_64 in this case (the page is not
> compound anymore, ->compound_head was already cleared before). As a result,
> page->mapping will be cleared before doing the checks in free_pages_check().
> 
> Not sure if the bogus "PageAnon() returning true" on x86_64 for the first
> tail page of a gigantic page (at this stage) has other theoretical
> implications, but they would also be fixed with this patch.
> 
> Signed-off-by: Gerald Schaefer <gerald.schaefer@xxxxxxxxxx>

Thanks Gerald, I agree with your fix.
Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>

However, like you I was wondering if this had any other implications.  I've
been examining code and can not find other places where this could be an
issue.  I did not find any issues, and in general since this is/was a huge
page, nobody should be doing PageAnon() on the tail pages except in a tear
down operation like this.

It would be great if someone with more page counting experience could
comment on this.

-- 
Mike Kravetz

> ---
>  mm/hugetlb.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index e197cd7..b64f8b7 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1030,6 +1030,7 @@ static void destroy_compound_gigantic_page(struct page *page,
>  	int nr_pages = 1 << order;
>  	struct page *p = page + 1;
>  
> +	atomic_set(compound_mapcount_ptr(page), 0);
>  	for (i = 1; i < nr_pages; i++, p = mem_map_next(p, page, i)) {
>  		clear_compound_head(p);
>  		set_page_refcounted(p);
>