linux-kernel - Re: [PATCH v3 2/2] mm: Make alloc_contig

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Date:   Fri, 26 Feb 2021 09:46:57 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        David Hildenbrand <david@...hat.com>,
        Muchun Song <songmuchun@...edance.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] mm: Make alloc_contig_range handle in-use hugetlb
 pages

On Mon 22-02-21 14:51:37, Oscar Salvador wrote:
[...]
> @@ -2394,9 +2397,19 @@ bool isolate_or_dissolve_huge_page(struct page *page)
>  	 */
>  	if (hstate_is_gigantic(h))
>  		return ret;
> -
> -	if (!page_count(head) && alloc_and_dissolve_huge_page(h, head))
> +retry:
> +	if (page_count(head) && isolate_huge_page(head, list)) {
>  		ret = true;
> +	} else if (!page_count(head)) {

This is rather head spinning. Do we need to test page_count in the else
branch? Do you want to optimize for a case where the page cannot be
isolated because of page_huge_active?

> +		int err = alloc_and_dissolve_huge_page(h, head);
> +
> +		if (!err) {
> +			ret = true;
> +		} else if (err == -EBUSY && try_again) {
> +			try_again = false;
> +			goto retry;
> +		}

Is this retry once logic really needed? Does it really give us any real
benefit? alloc_and_dissolve_huge_page already retries when the page is
being freed.

-- 
Michal Hocko
SUSE Labs