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Message-ID: <2b66797b-3ee9-a70f-25d0-dcab830859d2@redhat.com>
Date:   Wed, 19 Jul 2023 09:51:48 +0300
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Subject: Re: [PATCH] mm: hugetlb_vmemmap: use PageCompound() instead of
 PageReserved()
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To:     Muchun Song <songmuchun@bytedance.com>, mike.kravetz@oracle.com,
        muchun.song@linux.dev, akpm@linux-foundation.org
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org
References: <20230719063132.37676-1-songmuchun@bytedance.com>
From:   =?UTF-8?Q?Mika_Penttil=c3=a4?= <mpenttil@redhat.com>
In-Reply-To: <20230719063132.37676-1-songmuchun@bytedance.com>
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Hi,


On 19.7.2023 9.31, Muchun Song wrote:
> The ckeck of PageReserved() is easy to be broken in the future, PageCompound()
> is more stable to check if the page should be split.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>   mm/hugetlb_vmemmap.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index 4b9734777f69..8068fe890f52 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -73,8 +73,10 @@ static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start)
>   		 * be treated as indepdenent small pages (as they can be freed
>   		 * individually).
>   		 */
> -		if (!PageReserved(head))
> +		if (PageCompound(head)) {
> +			VM_BUG_ON(compound_order(head) != get_order(PMD_SIZE));
>   			split_page(head, get_order(PMD_SIZE));

I think vmemmap pages are not compound pages (even order > 0).
They would bug on here trying to split_page() :


void split_page(struct page *page, unsigned int order)
{
         int i;

         VM_BUG_ON_PAGE(PageCompound(page), page);


> +		}
>   
>   		/* Make pte visible before pmd. See comment in pmd_install(). */
>   		smp_wmb();

--Mika