linux-kernel - Re: [mm-unstable PATCH v5 1/8] mm/hugetlb: check gigantic_page_runtime_supported() in return_unused_surplus

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Date:   Mon, 11 Jul 2022 09:55:17 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Naoya Horiguchi <naoya.horiguchi@...ux.dev>, <linux-mm@...ck.org>
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>
Subject: Re: [mm-unstable PATCH v5 1/8] mm/hugetlb: check
 gigantic_page_runtime_supported() in return_unused_surplus_pages()

On 2022/7/8 13:36, 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 surprising (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
> at that time the gigantic pages were not supposed to be allocated/freed
> at run-time.  Now kernel can support runtime allocation/free, so let's
> check gigantic_page_runtime_supported() together.
> 
> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@....com>

Looks good to me. Thanks.

Reviewed-by: Miaohe Lin <linmiaohe@...wei.com>