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 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?


>
> b) dissolve_free_huge_pages() will fail if going below the reservation.
> In that case we really want to migrate free pages. This even applies to
> memory offlining.
>
> Either I am missing something important or this patch is more dangerous
> than it looks like.
>
> --
> Thanks,
>
> David / dhildenb
>