linux-kernel - Re: [PATCH v3 1/5] hugetlb: add demote hugetlb page sysfs interfaces

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Date:   Mon, 4 Oct 2021 15:00:59 +0200
From:   Oscar Salvador <osalvador@...e.de>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        David Hildenbrand <david@...hat.com>,
        Michal Hocko <mhocko@...e.com>, Zi Yan <ziy@...dia.com>,
        Muchun Song <songmuchun@...edance.com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        David Rientjes <rientjes@...gle.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 1/5] hugetlb: add demote hugetlb page sysfs interfaces

On Fri, Oct 01, 2021 at 10:52:06AM -0700, Mike Kravetz wrote:
> -which function as described above for the default huge page-sized case.
> +The demote interfaces provide the ability to split a huge page into
> +smaller huge pages.  For example, the x86 architecture supports both
> +1GB and 2MB huge pages sizes.  A 1GB huge page can be split into 512
> +2MB huge pages.  Demote interfaces are not available for the smallest
> +huge page size.  The demote interfaces are:
> +
> +demote_size
> +        is the size of demoted pages.  When a page is demoted a corresponding
> +        number of huge pages of demote_size will be created.  By default,
> +        demote_size is set to the next smaller huge page size.  If there are
> +        multiple smaller huge page sizes, demote_size can be set to any of
> +        these smaller sizes.  Only huge page sizes less then the current huge

s/then/than ?

>  static void __init hugetlb_init_hstates(void)
>  {
> -	struct hstate *h;
> +	struct hstate *h, *h2;
>  
>  	for_each_hstate(h) {
>  		if (minimum_order > huge_page_order(h))
> @@ -2995,6 +2995,23 @@ static void __init hugetlb_init_hstates(void)
>  		/* oversize hugepages were init'ed in early boot */
>  		if (!hstate_is_gigantic(h))
>  			hugetlb_hstate_alloc_pages(h);
> +
> +		/*
> +		 * Set demote order for each hstate.  Note that
> +		 * h->demote_order is initially 0.
> +		 * - We can not demote gigantic pages if runtime freeing
> +		 *   is not supported.
> +		 */
> +		if (!hstate_is_gigantic(h) ||
> +		    gigantic_page_runtime_supported()) {

Based on the comment, I think that making the condition explicit looks
better from my point of view.

if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
    continue;

> +			for_each_hstate(h2) {
> +				if (h2 == h)
> +					continue;
> +				if (h2->order < h->order &&
> +				    h2->order > h->demote_order)
> +					h->demote_order = h2->order;
> +			}
> +		}
>  	}
>  	VM_BUG_ON(minimum_order == UINT_MAX);
>  }
> @@ -3235,9 +3252,29 @@ static int set_max_huge_pages(struct hstate *h, unsigned long count, int nid,
>  	return 0;
>  }
>  

-- 
Oscar Salvador
SUSE L3