linux-kernel - Re: [PATCH v7 06/15] mm/hugetlb: Disable freeing vmemmap if struct page size is not power of two

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Date:   Wed, 9 Dec 2020 10:57:33 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Muchun Song <songmuchun@...edance.com>, corbet@....net,
        mike.kravetz@...cle.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, x86@...nel.org, hpa@...or.com,
        dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
        viro@...iv.linux.org.uk, akpm@...ux-foundation.org,
        paulmck@...nel.org, mchehab+huawei@...nel.org,
        pawan.kumar.gupta@...ux.intel.com, rdunlap@...radead.org,
        oneukum@...e.com, anshuman.khandual@....com, jroedel@...e.de,
        almasrymina@...gle.com, rientjes@...gle.com, willy@...radead.org,
        osalvador@...e.de, mhocko@...e.com, song.bao.hua@...ilicon.com
Cc:     duanxiongchun@...edance.com, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v7 06/15] mm/hugetlb: Disable freeing vmemmap if struct
 page size is not power of two

On 30.11.20 16:18, Muchun Song wrote:
> We only can free the tail vmemmap pages of HugeTLB to the buddy allocator
> when the size of struct page is a power of two.
> 
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> ---
>  mm/hugetlb_vmemmap.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index 51152e258f39..ad8fc61ea273 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -111,6 +111,11 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
>  	unsigned int nr_pages = pages_per_huge_page(h);
>  	unsigned int vmemmap_pages;
>  
> +	if (!is_power_of_2(sizeof(struct page))) {
> +		pr_info("disable freeing vmemmap pages for %s\n", h->name);

I'd just drop that pr_info(). Users are able to observe that it's
working (below), so they are able to identify that it's not working as well.

> +		return;
> +	}
> +
>  	vmemmap_pages = (nr_pages * sizeof(struct page)) >> PAGE_SHIFT;
>  	/*
>  	 * The head page and the first tail page are not to be freed to buddy
> 

Please squash this patch into the enabling patch and add a comment
instead, like

/* We cannot optimize if a "struct page" crosses page boundaries. */

-- 
Thanks,

David / dhildenb