Re: [PATCH v2 2/5] hugetlb: convert page_huge_active() HPageMigratable flag

Mike Kravetz
2021-01-22T23:15:06Z

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@huawei.com>
>