linux-kernel - Re: [PATCH v3 18/25] mm: Allow large pages to be added to the page cache

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Date:   Wed, 6 May 2020 11:32:10 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 18/25] mm: Allow large pages to be added to the page cache

On Sun, May 3, 2020 at 8:10 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Wed, Apr 29, 2020 at 06:36:50AM -0700, Matthew Wilcox wrote:
> > @@ -886,7 +906,7 @@ static int __add_to_page_cache_locked(struct page *page,
> >       /* Leave page->index set: truncation relies upon it */
> >       if (!huge)
> >               mem_cgroup_cancel_charge(page, memcg, false);
> > -     put_page(page);
> > +     page_ref_sub(page, nr);
> >       return xas_error(&xas);
> >  }
> >  ALLOW_ERROR_INJECTION(__add_to_page_cache_locked, ERRNO);
>
> This is wrong.  page_ref_sub() will not call __put_page() if the refcount
> gets to zero.  What do people prefer?
>
> -       put_page(page);
>
> (a)
> +       put_thp(page);
>
> (b)
> +       put_page_nr(page, nr);
>
> (c)
> +       if (page_ref_sub_return(page, nr) == 0)
> +               __put_page(page);

b or c IMHO. The shmem uses page_ref_add/page_ref_sub so we'd better
follow it. If go with b, it sounds better to add get_page_nr() as
well.

>