Re: [PATCH v3 2/5] mm/hugetlb: use nth_page() in place of direct struct page manipulation.

From: Zi Yan <ziy@nvidia.com>
To: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	linux-mips@vger.kernel.org
Cc: Zi Yan <ziy@nvidia.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
	"\"Matthew Wilcox (Oracle)\"" <willy@infradead.org>,
	David Hildenbrand <david@redhat.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Muchun Song <muchun.song@linux.dev>,
	"\"Mike Rapoport (IBM)\"" <rppt@kernel.org>,
	stable@vger.kernel.org, Muchun Song <songmuchun@bytedance.com>
Subject: Re: [PATCH v3 2/5] mm/hugetlb: use nth_page() in place of direct struct page manipulation.
Date: Wed, 13 Sep 2023 22:20:31 -0400	[thread overview]
Message-ID: <E2276305-4E00-4573-B8FE-9E598457FD48@nvidia.com> (raw)
In-Reply-To: <20230913201248.452081-3-zi.yan@sent.com>

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On 13 Sep 2023, at 16:12, Zi Yan wrote:

> From: Zi Yan <ziy@nvidia.com>
>
> When dealing with hugetlb pages, manipulating struct page pointers
> directly can get to wrong struct page, since struct page is not guaranteed
> to be contiguous on SPARSEMEM without VMEMMAP. Use nth_page() to handle
> it properly.
>
> Fixes: 57a196a58421 ("hugetlb: simplify hugetlb handling in follow_page_mask")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> Reviewed-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  mm/hugetlb.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index af74e83d92aa..8e68e6c53e66 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6469,7 +6469,7 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
>  			}
>  		}
>
> -		page += ((address & ~huge_page_mask(h)) >> PAGE_SHIFT);
> +		page = nth_page(page, ((address & ~huge_page_mask(h)) >> PAGE_SHIFT));
>
>  		/*
>  		 * Note that page may be a sub-page, and with vmemmap
> -- 
> 2.40.1

A wrong or non-existing page might be tried to be grabbed, either leading to
a non freeable page or kernel memory access errors. No bug is reported.
It comes from code inspection.

--
Best Regards,
Yan, Zi

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