linux-kernel - Re: [PATCH v2 2/5] hugetlb: convert page_huge

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Date:   Fri, 22 Jan 2021 15:12:19 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Muchun Song <songmuchun@...edance.com>,
        David Hildenbrand <david@...hat.com>,
        Oscar Salvador <osalvador@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 2/5] hugetlb: convert page_huge_active()
 HPageMigratable flag

On 1/21/21 10:53 PM, Miaohe Lin wrote:
> Hi:
> On 2021/1/20 9:30, Mike Kravetz wrote:
>> Use the new hugetlb page specific flag HPageMigratable to replace the
>> page_huge_active interfaces.  By it's name, page_huge_active implied
>> that a huge page was on the active list.  However, that is not really
>> what code checking the flag wanted to know.  It really wanted to determine
>> if the huge page could be migrated.  This happens when the page is actually
>> added the page cache and/or task page table.  This is the reasoning behind
> 
> s/added/added to/

Thanks

> 
>> the name change.
>>
...
>> @@ -1260,7 +1260,13 @@ static int scan_movable_pages(unsigned long start, unsigned long end,
>>  		if (!PageHuge(page))
>>  			continue;
>>  		head = compound_head(page);
>> -		if (page_huge_active(head))
>> +		/*
>> +		 * This test is racy as we hold no reference or lock.  The
>> +		 * hugetlb page could have been free'ed and head is no longer
>> +		 * a hugetlb page before the following check.  In such unlikely
>> +		 * cases false positives and negatives are possible.
>> +		 */
> 
> Is it necessary to mention that: "offline_pages() could handle these racy cases." in the comment ?
> 

I will enhance the comment to say that calling code must deal with these
possible scenarios.
-- 
Mike Kravetz

>> +		if (HPageMigratable(head))
>>  			goto found;
>>  		skip = compound_nr(head) - (page - head);
>>  		pfn += skip - 1;
>>
> 
> Looks good to me. Thanks.
> 
> Reviewed-by: Miaohe Lin <linmiaohe@...wei.com>
>