Re: [RFC] mm/thp: Update mm's MM_ANONPAGES stat in set_huge_zero_page()

[Yang Shi <shy828301@gmail.com>]

On Thu, May 13, 2021 at 9:50 AM Yang Shi <shy828301@gmail.com> wrote:
>
> On Thu, May 13, 2021 at 12:20 AM Anshuman Khandual
> <anshuman.khandual@arm.com> wrote:
> >
> > Although the zero huge page is being shared across various processes, each
> > mapping needs to update its mm's MM_ANONPAGES stat by HPAGE_PMD_NR in order
> > to be consistent. This just updates the stats in set_huge_zero_page() after
> > the mapping gets created.
>
> I don't get why MM_ANONPAGES needs to be inc'ed when huge zero page is
> installed. This may cause inconsistency between some counters, for
> example, MM_ANONPAGES may be much bigger than anon LRU.
>
> MM_ANONPAGES should not be inc'ed unless a new page is allocated and
> installed, right?

I just realized I mixed MM_ANONPAGES up with the global anon pages
counter. Take back my comment. Sorry for the confusion.

>
> >
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Zi Yan <ziy@nvidia.com>
> > Cc: linux-mm@kvack.org
> > Cc: linux-kernel@vger.kernel.org
> > Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> > ---
> > Should it update MM_SHMEM_PAGES instead ? Applies on latest mainline.
> >
> >  mm/huge_memory.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 63ed6b25deaa..262703304807 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -706,6 +706,7 @@ static void set_huge_zero_page(pgtable_t pgtable, struct mm_struct *mm,
> >         if (pgtable)
> >                 pgtable_trans_huge_deposit(mm, pmd, pgtable);
> >         set_pmd_at(mm, haddr, pmd, entry);
> > +       add_mm_counter(mm, MM_ANONPAGES, HPAGE_PMD_NR);
> >         mm_inc_nr_ptes(mm);
> >  }
> >
> > --
> > 2.20.1
> >
> >