Re: [External] Re: [PATCH v3 1/6] mm: migrate: do not migrate HugeTLB page whose refcount is one

[Muchun Song]

On Tue, Jan 12, 2021 at 9:51 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 12.01.21 14:40, Muchun Song wrote:
> > On Tue, Jan 12, 2021 at 7:11 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >>
> >> On 12.01.21 12:00, David Hildenbrand wrote:
> >>> On 10.01.21 13:40, Muchun Song wrote:
> >>>> If the refcount is one when it is migrated, it means that the page
> >>>> was freed from under us. So we are done and do not need to migrate.
> >>>>
> >>>> This optimization is consistent with the regular pages, just like
> >>>> unmap_and_move() does.
> >>>>
> >>>> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> >>>> Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> >>>> Acked-by: Yang Shi <shy828301@xxxxxxxxx>
> >>>> ---
> >>>>  mm/migrate.c | 6 ++++++
> >>>>  1 file changed, 6 insertions(+)
> >>>>
> >>>> diff --git a/mm/migrate.c b/mm/migrate.c
> >>>> index 4385f2fb5d18..a6631c4eb6a6 100644
> >>>> --- a/mm/migrate.c
> >>>> +++ b/mm/migrate.c
> >>>> @@ -1279,6 +1279,12 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
> >>>>              return -ENOSYS;
> >>>>      }
> >>>>
> >>>> +    if (page_count(hpage) == 1) {
> >>>> +            /* page was freed from under us. So we are done. */
> >>>> +            putback_active_hugepage(hpage);
> >>>> +            return MIGRATEPAGE_SUCCESS;
> >>>> +    }
> >>>> +
> >>>>      new_hpage = get_new_page(hpage, private);
> >>>>      if (!new_hpage)
> >>>>              return -ENOMEM;
> >>>>
> >>>
> >>> Question: What if called via alloc_contig_range() where we even want to
> >>> "migrate" free pages, meaning, relocate it?
> >>>
> >>
> >> To be more precise:
> >>
> >> a) We don't have dissolve_free_huge_pages() calls on the
> >> alloc_contig_range() path. So we *need* migration IIUC.
> >
> > Without this patch, if you want to migrate a HUgeTLB page,
> > the page is freed to the hugepage pool. With this patch,
> > the page is also freed to the hugepage pool.
> > I didn't see any different. I am missing something?
>
> I am definitely not an expert on hugetlb pools, that's why I am asking.
>
> Isn't it, that with your code, no new page is allocated - so
> dissolve_free_huge_pages() might just refuse to dissolve due to
> reservations, bailing out, no?

Without this patch, the new page can be allocated from the
hugepage pool. The dissolve_free_huge_pages() also
can refuse to dissolve due to reservations. Right?

>
> (as discussed, looks like alloc_contig_range() needs to be fixed to
> handle this correctly)
>
> --
> Thanks,
>
> David / dhildenb
>