Re: [PATCH] mm: hugetlb: fix missing put_page in gather_surplus_pages()

[Mike Kravetz <mike.kravetz@oracle.com>]

On 1/25/21 7:10 PM, Muchun Song wrote:
> The VM_BUG_ON_PAGE avoids the generation of any code, even if that
> expression has side-effects when !CONFIG_DEBUG_VM.
> 
> Fixes: e5dfacebe4a4 ("mm/hugetlb.c: just use put_page_testzero() instead of page_count()")
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> Cc: <stable@vger.kernel.org>
> ---
>  mm/hugetlb.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)

Thanks for finding and fixing this!  My bad for not noticing when the bug
was introduced.

> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index a6bad1f686c5..082ed643020b 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2047,13 +2047,16 @@ static int gather_surplus_pages(struct hstate *h, long delta)
>  
>  	/* Free the needed pages to the hugetlb pool */
>  	list_for_each_entry_safe(page, tmp, &surplus_list, lru) {
> +		int zeroed;
> +
>  		if ((--needed) < 0)
>  			break;
>  		/*
>  		 * This page is now managed by the hugetlb allocator and has
>  		 * no users -- drop the buddy allocator's reference.
>  		 */
> -		VM_BUG_ON_PAGE(!put_page_testzero(page), page);
> +		zeroed = put_page_testzero(page);
> +		VM_BUG_ON_PAGE(!zeroed, page);
>  		enqueue_huge_page(h, page);

I was wondering why this was not causing any problems.  We are putting the
hugetlb page on the free list with a count of 1.  There is no check in the
enqueue code.  When we dequeue the page, set_page_refcounted() is used to
set the count to 1 without looking at the current value.  And, all the other
VM_DEBUG macros are off so we mostly do not notice the bug.

Thanks again,

Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>

-- 
Mike Kravetz

>  	}
>  free:
>