linux-kernel - Re: [PATCH v2 1/3] mm: hugetlb: disable freeing vmemmap pages when struct page crosses page boundaries

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Date:   Wed, 2 Mar 2022 16:25:24 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Muchun Song <songmuchun@...edance.com>, corbet@....net,
        akpm@...ux-foundation.org, mcgrof@...nel.org,
        keescook@...omium.org, yzaikin@...gle.com
Cc:     linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, duanxiongchun@...edance.com, smuchun@...il.com
Subject: Re: [PATCH v2 1/3] mm: hugetlb: disable freeing vmemmap pages when
 struct page crosses page boundaries

On 3/2/22 00:37, Muchun Song wrote:
> If CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON is enabled and the size
> of "struct page" is not power of two, we cannot optimize vmemmap pages
> of HugeTLB pages. We should disable this feature in this case.

I'll let you reply to the question from Luis, but IIUC there is no issue
today as "struct page" is certainly a power of two.  This is more future
looking.  Correct?

> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> ---
>  mm/hugetlb_vmemmap.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index b3118dba0518..836d1117f08b 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -121,6 +121,17 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
>  	if (!hugetlb_free_vmemmap_enabled())
>  		return;
>  
> +	if (IS_ENABLED(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON) &&
> +	    !is_power_of_2(sizeof(struct page))) {
> +		/*
> +		 * The hugetlb_free_vmemmap_enabled_key can be enabled when
> +		 * CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON. It should
> +		 * be disabled if "struct page" crosses page boundaries.
> +		 */
> +		static_branch_disable(&hugetlb_free_vmemmap_enabled_key);

Should we possibly print a warning here as in the routine early_hugetlb_free_vmemmap_param?  This is called once per hstate, so
perhaps pr_warn_once.

-- 
Mike Kravetz

> +		return;
> +	}
> +
>  	vmemmap_pages = (nr_pages * sizeof(struct page)) >> PAGE_SHIFT;
>  	/*
>  	 * The head page is not to be freed to buddy allocator, the other tail