linux-kernel - Re: [PATCH 1/6] mm/hugetlb: fix incorrect update of max_huge

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Message-ID: <5eebaeff-c002-2a1e-b5bb-fffe7e987ca0@huawei.com>
Date:   Wed, 17 Aug 2022 09:53:47 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     <songmuchun@...edance.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/6] mm/hugetlb: fix incorrect update of max_huge_pages

On 2022/8/17 7:34, Mike Kravetz wrote:
> On 08/16/22 16:20, Andrew Morton wrote:
>> On Tue, 16 Aug 2022 15:52:47 -0700 Mike Kravetz <mike.kravetz@...cle.com> wrote:
>>
>>> On 08/16/22 21:05, Miaohe Lin wrote:
>>>> There should be pages_per_huge_page(h) / pages_per_huge_page(target_hstate)
>>>> pages incremented for target_hstate->max_huge_pages when page is demoted.
>>>> Update max_huge_pages accordingly for consistency.
>>>>
>>>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>>>> ---
>>>>  mm/hugetlb.c | 3 ++-
>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>>> index ea1c7bfa1cc3..e72052964fb5 100644
>>>> --- a/mm/hugetlb.c
>>>> +++ b/mm/hugetlb.c
>>>> @@ -3472,7 +3472,8 @@ static int demote_free_huge_page(struct hstate *h, struct page *page)
>>>>  	 * based on pool changes for the demoted page.
>>>>  	 */
>>>>  	h->max_huge_pages--;
>>>> -	target_hstate->max_huge_pages += pages_per_huge_page(h);
>>>> +	target_hstate->max_huge_pages +=
>>>> +		pages_per_huge_page(h) / pages_per_huge_page(target_hstate);
>>>
>>> Thanks!
>>>
>>> That is indeed incorrect.  However the miscalculation should not have any 
>>> consequences.  Correct?  The value is used when initially populating the
>>> pools.  It is never read and used again.  It is written to in
>>> set_max_huge_pages if someone changes the number of hugetlb pages.
>>>
>>> I guess that is a long way of saying I am not sure why we care about trying
>>> to keep max_huge_pages up to date?  I do not think it matters.
>>>
>>> I also thought, if we are going to adjust max_huge_pages here we may
>>> also want to adjust the node specific value: h->max_huge_pages_node[node].
>>> There are a few other places where the global max_huge_pages is adjusted
>>> without adjusting the node specific value.
>>>
>>> The more I think about it, the more I think we should explore just
>>> eliminating any adjustment of this/these values after initially
>>> populating the pools.
>>
>> I'm thinking we should fix something that is "indeed incorrect" before
>> going on to more extensive things?
> 
> Sure, I am good with that.
> 
> Just wanted to point out that the incorrect calculation does not have
> any negative consequences.  Maybe prompting Miaohe to look into the more
> extensive cleanup.

Many thanks both. I will try to do this "more extensive cleanup" after pending work is done.

Thanks,
Miaohe Lin