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    SubjectRe: [PATCH v2 2/6] mm: hugetlbfs: fix cannot migrate the fallocated HugeTLB page
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    On 1/6/21 12:02 PM, Michal Hocko wrote:
    > On Wed 06-01-21 11:30:25, Mike Kravetz wrote:
    >> On 1/6/21 8:35 AM, Michal Hocko wrote:
    >>> On Wed 06-01-21 16:47:35, Muchun Song wrote:
    >>>> Because we only can isolate a active page via isolate_huge_page()
    >>>> and hugetlbfs_fallocate() forget to mark it as active, we cannot
    >>>> isolate and migrate those pages.
    >>>
    >>> I've little bit hard time to understand this initially and had to dive
    >>> into the code to make sense of it. I would consider the following
    >>> wording easier to grasp. Feel free to reuse if you like.
    >>> "
    >>> If a new hugetlb page is allocated during fallocate it will not be
    >>> marked as active (set_page_huge_active) which will result in a later
    >>> isolate_huge_page failure when the page migration code would like to
    >>> move that page. Such a failure would be unexpected and wrong.
    >>> "
    >>>
    >>> Now to the fix. I believe that this patch shows that the
    >>> set_page_huge_active is just too subtle. Is there any reason why we
    >>> cannot make all freshly allocated huge pages active by default?
    >>
    >> I looked into that yesterday. The primary issue is in page fault code,
    >> hugetlb_no_page is an example. If page_huge_active is set, then it can
    >> be isolated for migration. So, migration could race with the page fault
    >> and the page could be migrated before being added to the page table of
    >> the faulting task. This was an issue when hugetlb_no_page set_page_huge_active
    >> right after allocating and clearing the huge page. Commit cb6acd01e2e4
    >> moved the set_page_huge_active after adding the page to the page table
    >> to address this issue.
    >
    > Thanks for the clarification. I was not aware of this subtlety. The
    > existing comment is not helping much TBH. I am still digesting the
    > suggested race. The page is new and exclusive and not visible via page
    > tables yet, so the only source of the migration would be pfn based
    > (hotplug, poisoning), right?

    That is correct.


    > Btw. s@set_page_huge_active@set_page_huge_migrateable@ would help
    > readability IMHO. With a comment explaining that this _has_ to be called
    > after the page is fully initialized.

    Agree, I will add that as a future enhancement.

    --
    Mike Kravetz

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