linux-kernel - Re: [PATCH] mm: mempolicy: don't have to split pmd for huge zero page

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Date:   Mon, 7 Jun 2021 10:00:01 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Zi Yan <ziy@...dia.com>, nao.horiguchi@...il.com,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: mempolicy: don't have to split pmd for huge zero page

On Sun, Jun 6, 2021 at 11:21 PM Michal Hocko <mhocko@...e.com> wrote:
>
> On Fri 04-06-21 13:35:13, Yang Shi wrote:
> > When trying to migrate pages to obey mempolicy, the huge zero page is
> > split then the page table walk at PTE level just skips zero page.  So it
> > seems pointless to split huge zero page, it could be just skipped like
> > base zero page.
>
> My THP knowledge is not the best but this is incorrect AIACS. Huge zero
> page is not split. We do split the pmd which is mapping the said page. I
> suspect you refer to vm_normal_page when talking about a zero page but
> please be aware that huge zero page is not a normal zero page. It is
> allocated dynamically (see get_huge_zero_page).

For a normal huge page, yes, split_huge_pmd() just splits pmd. But
actually the base zero pfn will be inserted to PTEs when splitting
huge zero pmd. Please check __split_huge_zero_page_pmd() out.

I should make this point clearer in the commit log. Sorry for the confusion.

>
> So in the end you patch disables mbind of zero pages to a target node
> and that is a regression.

Do we really migrate zero page? IIUC zero page is just skipped by
vm_normal_page() check in queue_pages_pte_range(), isn't it?

>
> Have you tested the patch?

No, just build test. I thought this change was straightforward.

>
> > Set ACTION_CONTINUE to prevent the walk_page_range() split the pmd for
> > this case.
>
> Btw. this changelog is missing a problem statement. I suspect there is
> no actual problem that it should fix and it is likely driven by reading
> the code. Right?

The actual problem is it is pointless to split a huge zero pmd. Yes,
it is driven by visual inspection.

The behavior before the patch for huge zero page is:
split huge zero pmd (insert base zero pfn to ptes)
walk ptes
skip zero pfn

So why not just skip the huge zero page in the first place?

>
> > Signed-off-by: Yang Shi <shy828301@...il.com>
> > ---
> >  mm/mempolicy.c | 9 +++++----
> >  1 file changed, 5 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > index b5f4f584009b..205c1a768775 100644
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -436,7 +436,8 @@ static inline bool queue_pages_required(struct page *page,
> >
> >  /*
> >   * queue_pages_pmd() has four possible return values:
> > - * 0 - pages are placed on the right node or queued successfully.
> > + * 0 - pages are placed on the right node or queued successfully, or
> > + *     special page is met, i.e. huge zero page.
> >   * 1 - there is unmovable page, and MPOL_MF_MOVE* & MPOL_MF_STRICT were
> >   *     specified.
> >   * 2 - THP was split.
> > @@ -460,8 +461,7 @@ static int queue_pages_pmd(pmd_t *pmd, spinlock_t *ptl, unsigned long addr,
> >       page = pmd_page(*pmd);
> >       if (is_huge_zero_page(page)) {
> >               spin_unlock(ptl);
> > -             __split_huge_pmd(walk->vma, pmd, addr, false, NULL);
> > -             ret = 2;
> > +             walk->action = ACTION_CONTINUE;
> >               goto out;
> >       }
> >       if (!queue_pages_required(page, qp))
> > @@ -488,7 +488,8 @@ static int queue_pages_pmd(pmd_t *pmd, spinlock_t *ptl, unsigned long addr,
> >   * and move them to the pagelist if they do.
> >   *
> >   * queue_pages_pte_range() has three possible return values:
> > - * 0 - pages are placed on the right node or queued successfully.
> > + * 0 - pages are placed on the right node or queued successfully, or
> > + *     special page is met, i.e. zero page.
> >   * 1 - there is unmovable page, and MPOL_MF_MOVE* & MPOL_MF_STRICT were
> >   *     specified.
> >   * -EIO - only MPOL_MF_STRICT was specified and an existing page was already
> > --
> > 2.26.2
>
> --
> Michal Hocko
> SUSE Labs