linux-kernel - Re: [PATCH] mm/hugetlb: allow gigantic page allocation to migrate away smaller huge page

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Date:   Sun, 23 Jun 2019 22:03:42 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Pingfan Liu <kernelfans@...il.com>
Cc:     linux-mm@...ck.org, Mike Kravetz <mike.kravetz@...cle.com>,
        Oscar Salvador <osalvador@...e.de>,
        David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/hugetlb: allow gigantic page allocation to migrate
 away smaller huge page

On Mon, Jun 24, 2019 at 12:21:08PM +0800, Pingfan Liu wrote:
> The current pfn_range_valid_gigantic() rejects the pud huge page allocation
> if there is a pmd huge page inside the candidate range.
> 
> But pud huge resource is more rare, which should align on 1GB on x86. It is
> worth to allow migrating away pmd huge page to make room for a pud huge
> page.
> 
> The same logic is applied to pgd and pud huge pages.

I'm sorry but I don't quite understand why we should do this.  Is this a bug or
an optimization?  It sounds like an optimization.

> 
> Signed-off-by: Pingfan Liu <kernelfans@...il.com>
> Cc: Mike Kravetz <mike.kravetz@...cle.com>
> Cc: Oscar Salvador <osalvador@...e.de>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: linux-kernel@...r.kernel.org
> ---
>  mm/hugetlb.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index ac843d3..02d1978 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1081,7 +1081,11 @@ static bool pfn_range_valid_gigantic(struct zone *z,
>  			unsigned long start_pfn, unsigned long nr_pages)
>  {
>  	unsigned long i, end_pfn = start_pfn + nr_pages;
> -	struct page *page;
> +	struct page *page = pfn_to_page(start_pfn);
> +
> +	if (PageHuge(page))
> +		if (compound_order(compound_head(page)) >= nr_pages)

I don't think you want compound_order() here.

Ira

> +			return false;
>  
>  	for (i = start_pfn; i < end_pfn; i++) {
>  		if (!pfn_valid(i))
> @@ -1098,8 +1102,6 @@ static bool pfn_range_valid_gigantic(struct zone *z,
>  		if (page_count(page) > 0)
>  			return false;
>  
> -		if (PageHuge(page))
> -			return false;
>  	}
>  
>  	return true;
> -- 
> 2.7.5
>