Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Sat, 28 Mar 2020 15:27:35 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Yang Shi <shy828301@gmail.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH 5/7] khugepaged: Allow to collapse PTE-mapped compound
 pages
Message-ID: <20200328122735.nzius2ikvnyvpf2f@box>
References: <20200327170601.18563-1-kirill.shutemov@linux.intel.com>
 <20200327170601.18563-6-kirill.shutemov@linux.intel.com>
 <CAHbLzkoVq-ssduiPwdzcsL2bVhPwmw4X9ktAO0CYOVAi8H84oA@mail.gmail.com>
 <20200328003424.kusu2xnhnlbmnfzl@box>
 <CAHbLzkpV7=EGQVeEEZ_jhpWa-nnVkiZ4_Qa=0KoZCRntprWhgg@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAHbLzkpV7=EGQVeEEZ_jhpWa-nnVkiZ4_Qa=0KoZCRntprWhgg@mail.gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Fri, Mar 27, 2020 at 06:09:38PM -0700, Yang Shi wrote:
> On Fri, Mar 27, 2020 at 5:34 PM Kirill A. Shutemov <kirill@shutemov.name> wrote:
> >
> > On Fri, Mar 27, 2020 at 11:53:57AM -0700, Yang Shi wrote:
> > > On Fri, Mar 27, 2020 at 10:06 AM Kirill A. Shutemov
> > > <kirill@shutemov.name> wrote:
> > > >
> > > > We can collapse PTE-mapped compound pages. We only need to avoid
> > > > handling them more than once: lock/unlock page only once if it's present
> > > > in the PMD range multiple times as it handled on compound level. The
> > > > same goes for LRU isolation and putpack.
> > > >
> > > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > > > ---
> > > >  mm/khugepaged.c | 41 +++++++++++++++++++++++++++++++----------
> > > >  1 file changed, 31 insertions(+), 10 deletions(-)
> > > >
> > > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > > index b47edfe57f7b..c8c2c463095c 100644
> > > > --- a/mm/khugepaged.c
> > > > +++ b/mm/khugepaged.c
> > > > @@ -515,6 +515,17 @@ void __khugepaged_exit(struct mm_struct *mm)
> > > >
> > > >  static void release_pte_page(struct page *page)
> > > >  {
> > > > +       /*
> > > > +        * We need to unlock and put compound page on LRU only once.
> > > > +        * The rest of the pages have to be locked and not on LRU here.
> > > > +        */
> > > > +       VM_BUG_ON_PAGE(!PageCompound(page) &&
> > > > +                       (!PageLocked(page) && PageLRU(page)), page);
> > > > +
> > > > +       if (!PageLocked(page))
> > > > +               return;
> > > > +
> > > > +       page = compound_head(page);
> > > >         dec_node_page_state(page, NR_ISOLATED_ANON + page_is_file_cache(page));
> > >
> > > We need count in the number of base pages. The same counter is
> > > modified by vmscan in base page unit.
> >
> > Is it though? Where?
> 
> __mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, nr_taken) in
> vmscan.c, here nr_taken is nr_compound(page), so if it is THP the
> number would be 512.

Could you point a particular codepath?

> So in both inc and dec path of collapse PTE mapped THP, we should mod
> nr_compound(page) too.

I disagree. Compound page is represented by single entry on LRU, so it has
to be counted once.

-- 
 Kirill A. Shutemov