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

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Message-ID: <CAHbLzkr627vwDovZ23Ofje1vfu+m6BbYWs=tvptNpQwf1ramyQ@mail.gmail.com>
Date:   Fri, 10 Dec 2021 09:59:46 -0800
From:   Yang Shi <shy828301@...il.com>
To:     Zi Yan <ziy@...dia.com>
Cc:     Eric Ren <renzhengeek@...il.com>,
        David Hildenbrand <david@...hat.com>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        linuxppc-dev@...ts.ozlabs.org,
        virtualization@...ts.linux-foundation.org,
        iommu@...ts.linux-foundation.org, Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [RFC PATCH v2 3/7] mm: migrate: allocate the right size of non
 hugetlb or THP compound pages.

On Fri, Dec 10, 2021 at 8:08 AM Zi Yan <ziy@...dia.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@...dia.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@...dia.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