linux-kernel - Re: [PATCH v2 1/9] mm/hugetlb: remove checking hstate_is_gigantic() in return_unused_surplus

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Date:   Fri, 24 Jun 2022 10:25:48 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Naoya Horiguchi <nao.horiguchi@...il.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Liu Shixin <liushixin2@...wei.com>,
        Yang Shi <shy828301@...il.com>,
        Oscar Salvador <osalvador@...e.de>,
        Muchun Song <songmuchun@...edance.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        <linux-kernel@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH v2 1/9] mm/hugetlb: remove checking hstate_is_gigantic()
 in return_unused_surplus_pages()

On 2022/6/24 7:51, Naoya Horiguchi wrote:
> From: Naoya Horiguchi <naoya.horiguchi@....com>
> 
> I found a weird state of 1GB hugepage pool, caused by the following
> procedure:
> 
>   - run a process reserving all free 1GB hugepages,
>   - shrink free 1GB hugepage pool to zero (i.e. writing 0 to
>     /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages), then
>   - kill the reserving process.
> 
> , then all the hugepages are free *and* surplus at the same time.
> 
>   $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages
>   3
>   $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/free_hugepages
>   3
>   $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/resv_hugepages
>   0
>   $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/surplus_hugepages
>   3
> 
> This state is resolved by reserving and allocating the pages then
> freeing them again, so this seems not to result in serious problem.
> But it's a little surprizing (shrinking pool suddenly fails).
> 
> This behavior is caused by hstate_is_gigantic() check in
> return_unused_surplus_pages(). This was introduced so long ago in 2008
> by commit aa888a74977a ("hugetlb: support larger than MAX_ORDER"), and
> it seems to me that this check is no longer unnecessary. Let's remove it.

s/unnecessary/necessary/

> 
> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@....com>
> ---
>  mm/hugetlb.c | 4 ----
>  1 file changed, 4 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index a57e1be41401..c538278170a2 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2432,10 +2432,6 @@ static void return_unused_surplus_pages(struct hstate *h,
>  	/* Uncommit the reservation */
>  	h->resv_huge_pages -= unused_resv_pages;
>  
> -	/* Cannot return gigantic pages currently */
> -	if (hstate_is_gigantic(h))
> -		goto out;
> -

IIUC it might be better to do the below check:
	/*
	 * Cannot return gigantic pages currently if runtime gigantic page
	 * allocation is not supported.
	 */
	if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
		goto out;

But I might be miss something.

Thanks.

>  	/*
>  	 * Part (or even all) of the reservation could have been backed
>  	 * by pre-allocated pages. Only free surplus pages.
>