linux-kernel - Re: [PATCH 4/7] mm/khugepaged: minor cleanup for collapse

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Date:   Wed, 15 Jun 2022 11:18:35 -0700
From:   Yang Shi <shy828301@...il.com>
To:     "Zach O'Keefe" <zokeefe@...gle.com>
Cc:     Miaohe Lin <linmiaohe@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Howells <dhowells@...hat.com>, NeilBrown <neilb@...e.de>,
        Alistair Popple <apopple@...dia.com>,
        David Hildenbrand <david@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Peter Xu <peterx@...hat.com>, Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/7] mm/khugepaged: minor cleanup for collapse_file

On Wed, Jun 15, 2022 at 8:55 AM Zach O'Keefe <zokeefe@...gle.com> wrote:
>
> On 11 Jun 16:47, Miaohe Lin wrote:
> > nr_none is always 0 for non-shmem case because the page can be read from
> > the backend store. So when nr_none ! = 0, it must be in is_shmem case.
> > Also only adjust the nrpages and uncharge shmem when nr_none != 0 to save
> > cpu cycles.
> >
> > Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> > ---
> >  mm/khugepaged.c | 9 ++++-----
> >  1 file changed, 4 insertions(+), 5 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 1b5dd3820eac..8e6fad7c7bd9 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -1885,8 +1885,7 @@ static void collapse_file(struct mm_struct *mm,
> >
> >       if (nr_none) {
> >               __mod_lruvec_page_state(new_page, NR_FILE_PAGES, nr_none);
> > -             if (is_shmem)
> > -                     __mod_lruvec_page_state(new_page, NR_SHMEM, nr_none);
> > +             __mod_lruvec_page_state(new_page, NR_SHMEM, nr_none);
> >       }
>
>
> Might be worth a small comment here - even though folks can see in above code
> that this is only incremented in shmem path, might be nice to say why it's
> always 0 for non-shmem (or conversely, why it's only possible to be non 0 on
> shmem).

Agreed, better to have some comments in the code.

>
> >
> >       /* Join all the small entries into a single multi-index entry */
> > @@ -1950,10 +1949,10 @@ static void collapse_file(struct mm_struct *mm,
> >
> >               /* Something went wrong: roll back page cache changes */
> >               xas_lock_irq(&xas);
> > -             mapping->nrpages -= nr_none;
> > -
> > -             if (is_shmem)
> > +             if (nr_none) {
> > +                     mapping->nrpages -= nr_none;
> >                       shmem_uncharge(mapping->host, nr_none);
> > +             }
> >
> >               xas_set(&xas, start);
> >               xas_for_each(&xas, page, end - 1) {
> > --
> > 2.23.0
> >
> >
>
> Otherwise,
>
> Reviewed-by: Zach O'Keefe <zokeefe@...gle.com>
>