[RFC PATCH v2 3/7] mm: migrate: allocate the right size of non hugetlb or THP compound pages.

[Yang Shi]

On Fri, Dec 10, 2021 at 8:08 AM Zi Yan <ziy at nvidia.com> wrote:
>
> On 10 Dec 2021, at 2:53, Eric Ren wrote:
>
> > Hi,
> >
> > On 2021/12/10 07:04, Zi Yan wrote:
> >> From: Zi Yan <ziy at nvidia.com>
> >>
> >> alloc_migration_target() is used by alloc_contig_range() and non-LRU
> >> movable compound pages can be migrated. Current code does not allocate the
> >> right page size for such pages. Check THP precisely using
> >> is_transparent_huge() and add allocation support for non-LRU compound
> >> pages.
> > Could you elaborate on why the current code doesn't get the right size?  how this patch fixes it.
>
> The current code only check PageHuge() and PageTransHuge(). Non-LRU compound
> pages will be regarded as PageTransHuge() and an order-9 page is always allocated
> regardless of the actual page order. This patch makes the exact check for
> THPs using is_transparent_huge() instead of PageTransHuge() and checks PageCompound()
> after PageHuge() and is_transparent_huge() for non-LRU compound pages.
>
> >
> > The description sounds like it's an existing bug, if so, the patch subject should be changed to
> > "Fixes ..."?
>
> I have not seen any related bug report.
>
> >
> >>
> >> Signed-off-by: Zi Yan <ziy at nvidia.com>
> >> ---
> >>   mm/migrate.c | 8 ++++++--
> >>   1 file changed, 6 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/mm/migrate.c b/mm/migrate.c
> >> index d487a399253b..2ce3c771b1de 100644
> >> --- a/mm/migrate.c
> >> +++ b/mm/migrate.c
> >> @@ -1563,7 +1563,7 @@ struct page *alloc_migration_target(struct page *page, unsigned long private)
> >>              return alloc_huge_page_nodemask(h, nid, mtc->nmask, gfp_mask);
> >>      }
> >>  -   if (PageTransHuge(page)) {
> >> +    if (is_transparent_hugepage(page)) {
> >>              /*
> >>               * clear __GFP_RECLAIM to make the migration callback
> >>               * consistent with regular THP allocations.
> >> @@ -1572,13 +1572,17 @@ struct page *alloc_migration_target(struct page *page, unsigned long private)
> > if (PageTransHuge(page)) {  // just give more code context
> > ...
> >>              gfp_mask |= GFP_TRANSHUGE;
> >>              order = HPAGE_PMD_ORDER;
> > order assigned here
> >>      }
> >> +    if (PageCompound(page)) {
> >> +            gfp_mask |= __GFP_COMP;
> >> +            order = compound_order(page);
> > re-assinged again as THP is a compound page?
>
> Ah, you are right. Will use else if instead. Thanks.

You don't have to use else if, you could just do:

if (PageCompound(page)) {
    gfp_mask |= __GFP_COMP;
    order = compound_order(page);
}

if (is_transparent_hugepage(page)) {
    /* Manipulate THP specific gfp mask */
    ....
}


>
> > Thanks,
> > Eric
> >> +    }
> >>      zidx = zone_idx(page_zone(page));
> >>      if (is_highmem_idx(zidx) || zidx == ZONE_MOVABLE)
> >>              gfp_mask |= __GFP_HIGHMEM;
> >>      new_page = __alloc_pages(gfp_mask, order, nid, mtc->nmask);
> >>  -   if (new_page && PageTransHuge(new_page))
> >> +    if (new_page && is_transparent_hugepage(page))
> >>              prep_transhuge_page(new_page);
> >>      return new_page;
>
>
> --
> Best Regards,
> Yan, Zi