Re: [PATCH] hugetlb: Fix __prep_compound_gigantic_page page flag setting

[Tarun Sahu <tsahu@linux.ibm.com>]

Hi,

Tested it on top of v6.1.0-rc5
with 1G Hugepages on PPC64le radix MMU.

Tested-by: Tarun Sahu <tsahu@linux.ibm.com>


~Tarun

On Nov 18 2022, Mike Kravetz wrote:
> Commit 2b21624fc232 ("hugetlb: freeze allocated pages before creating
> hugetlb pages") changed the order page flags were cleared and set in the
> head page.  It moved the __ClearPageReserved after __SetPageHead.
> However, there is a check to make sure __ClearPageReserved is never
> done on a head page.  If CONFIG_DEBUG_VM_PGFLAGS is enabled, the
> following BUG will be hit when creating a hugetlb gigantic page:
> 
>     page dumped because: VM_BUG_ON_PAGE(1 && PageCompound(page))
>     ------------[ cut here ]------------
>     kernel BUG at include/linux/page-flags.h:500!
>     Call Trace will differ depending on whether hugetlb page is created
>     at boot time or run time.
> 
> Make sure to __ClearPageReserved BEFORE __SetPageHead.
> 
> Reported-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
> Fixes: 2b21624fc232 ("hugetlb: freeze allocated pages before creating hugetlb pages")
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
> ---
>  mm/hugetlb.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index e48f8ef45b17..f1385c3b6c96 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1800,6 +1800,7 @@ static bool __prep_compound_gigantic_page(struct page *page, unsigned int order,
>  
>  	/* we rely on prep_new_huge_page to set the destructor */
>  	set_compound_order(page, order);
> +	__ClearPageReserved(page);
>  	__SetPageHead(page);
>  	for (i = 0; i < nr_pages; i++) {
>  		p = nth_page(page, i);
> @@ -1816,7 +1817,8 @@ static bool __prep_compound_gigantic_page(struct page *page, unsigned int order,
>  		 * on the head page when they need know if put_page() is needed
>  		 * after get_user_pages().
>  		 */
> -		__ClearPageReserved(p);
> +		if (i != 0)	/* head page cleared above */
> +			__ClearPageReserved(p);
>  		/*
>  		 * Subtle and very unlikely
>  		 *
> -- 
> 2.38.1
>