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 16:15:19 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Muchun Song <songmuchun@...edance.com>
CC:     Naoya Horiguchi <nao.horiguchi@...il.com>,
        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>,
        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 16:03, Muchun Song wrote:
> On Fri, Jun 24, 2022 at 10:25:48AM +0800, Miaohe Lin wrote:
>> 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;
>>
> 
> The change looks good to me. However, the comments above is unnecessary
> since gigantic_page_runtime_supported() is straightforward.

Agree. The comments can be removed.

> 
> Thanks.

Thanks for reviewing.

>  
>> 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.
>>>
>>
>>
> .
>