Re: [External] Re: [PATCH] mm: migrate: fix missing update page_private to hugetlb_page_subpool

[Muchun Song <songmuchun@bytedance.com>]

On Fri, May 21, 2021 at 2:17 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 5/19/21 10:13 PM, Muchun Song wrote:
> > On Thu, May 20, 2021 at 12:24 PM Andrew Morton
> > <akpm@linux-foundation.org> wrote:
> >>
> >> On Thu, 20 May 2021 10:59:49 +0800 Muchun Song <songmuchun@bytedance.com> wrote:
> >>
> >>> Since commit d6995da31122 ("hugetlb: use page.private for hugetlb specific
> >>> page flags") converts page.private for hugetlb specific page flags. We
> >>> should use hugetlb_page_subpool() to get the subpool pointer instead of
> >>> page_private(). The commit forgot to update it in the page migration
> >>> routine. So fix it.
> >>>
> >>> ...
> >>>
> >>> --- a/mm/migrate.c
> >>> +++ b/mm/migrate.c
> >>> @@ -1290,7 +1290,7 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
> >>>        * page_mapping() set, hugetlbfs specific move page routine will not
> >>>        * be called and we could leak usage counts for subpools.
> >>>        */
> >>> -     if (page_private(hpage) && !page_mapping(hpage)) {
> >>> +     if (hugetlb_page_subpool(hpage) && !page_mapping(hpage)) {
> >>>               rc = -EBUSY;
> >>>               goto out_unlock;
> >>>       }
> >>
> >> So it uses the wrong page*, so this isn't just a cosmetic fix.  One
> >> cannot tell from this changelog.
> >>
> >> Please describe the runtime effects of this bug.  Please always include
> >> this information when fixing bugs.  And when adding them.
> >
> > OK. I should update the commit log in the next version.
> >
>
> Unfortunately, it is more than a cosmetic fix.
>
> This 'could' prevent the migration of hugetlb pages.  page_private(hpage)
> is now used for hugetlb page specific flags.  At migration time, the
> only flag which could be set is HPageVmemmapOptimized.  This flag will
> only be set if the new vmemmap reduction feature is enabled.  In
> addition, !page_mapping() implies an anonymous mapping.  So, this will
> prevent migration of hugetb pages in anonymous mappings if the vmemmap
> reduction feature is enabled.
>
> In addition, that if statement checked for the rare race condition of a
> page being migrated while in the process of being freed.  Since that
> check is now wrong, we could leak hugetlb subpool usage counts.
>
> Muchun, feel free to use as much of that as you would like in the
> updated commit message.

Thanks Mike. I'll use this.

> --
> Mike Kravetz