linux-kernel - Re: [v3 PATCH 3/5] mm: hwpoison: refactor refcount check handling

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Message-ID: <CAHbLzkqtaF2iFwg0TmMm_1q+o+-O=CXAAPY2izxL6N=8umX_Cg@mail.gmail.com>
Date:   Wed, 6 Oct 2021 19:47:20 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     HORIGUCHI NAOYA(堀口 直也) 
        <naoya.horiguchi@....com>, Hugh Dickins <hughd@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [v3 PATCH 3/5] mm: hwpoison: refactor refcount check handling

On Wed, Oct 6, 2021 at 3:02 PM Peter Xu <peterx@...hat.com> wrote:
>
> On Thu, Sep 30, 2021 at 02:53:09PM -0700, Yang Shi wrote:
> > +/*
> > + * Return true if page is still referenced by others, otherwise return
> > + * false.
> > + *
> > + * The dec is true when one extra refcount is expected.
> > + */
> > +static bool has_extra_refcount(struct page_state *ps, struct page *p,
> > +                            bool dec)
>
> Nit: would it be nicer to keep using things like "extra_pins", so we pass in 1
> for swapcache dirty case and 0 for the rest?  Then it'll also match with most
> of the similar cases in e.g. huge_memory.c (please try grep "extra_pins" there).

Thanks for the suggestion. Yes, it makes some sense to me. And the
code comments in patch 4/5 does says (the suggested version by Naoya):

/*
 * The shmem page is kept in page cache instead of truncating
 * so is expected to have an extra refcount after error-handling.
 */

Will rename it in the new version.

>
> > +{
> > +     int count = page_count(p) - 1;
> > +
> > +     if (dec)
> > +             count -= 1;
> > +
> > +     if (count > 0) {
> > +             pr_err("Memory failure: %#lx: %s still referenced by %d users\n",
> > +                    page_to_pfn(p), action_page_types[ps->type], count);
> > +             return true;
> > +     }
> > +
> > +     return false;
> > +}
> > +
> >  /*
> >   * Error hit kernel page.
> >   * Do nothing, try to be lucky and not touch this instead. For a few cases we
> >   * could be more sophisticated.
> >   */
> > -static int me_kernel(struct page *p, unsigned long pfn)
> > +static int me_kernel(struct page_state *ps, struct page *p)
>
> Not sure whether it's intended, but some of the action() hooks do not call the
> refcount check now while in the past they'll all do.  Just to double check
> they're expected, like this one and me_unknown().

Yeah, it is intentional. Before this change all me_* handlers did
check refcount even though it was not necessary, for example,
me_kernel() and me_unknown().

>
> >  {
> >       unlock_page(p);
> >       return MF_IGNORED;
> > @@ -820,9 +852,9 @@ static int me_kernel(struct page *p, unsigned long pfn)
> >  /*
> >   * Page in unknown state. Do nothing.
> >   */
> > -static int me_unknown(struct page *p, unsigned long pfn)
> > +static int me_unknown(struct page_state *ps, struct page *p)
> >  {
> > -     pr_err("Memory failure: %#lx: Unknown page state\n", pfn);
> > +     pr_err("Memory failure: %#lx: Unknown page state\n", page_to_pfn(p));
> >       unlock_page(p);
> >       return MF_FAILED;
> >  }
>
> Thanks,
>
> --
> Peter Xu
>