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References: <20230722231508.1030269-1-pasha.tatashin@soleen.com>
 <20230722231508.1030269-2-pasha.tatashin@soleen.com> <ZLyI48BYnecaROmZ@casper.infradead.org>
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From:   Pasha Tatashin <pasha.tatashin@soleen.com>
Date:   Sat, 22 Jul 2023 23:35:12 -0400
Message-ID: <CA+CK2bANEkfEe_qMTivTyAhFd8+bi_YmQknJOtep2Q51xpdkLg@mail.gmail.com>
Subject: Re: [PATCH v2 1/3] mm/page_table_check: Do WARN_ON instead of BUG_ON
To:     Matthew Wilcox <willy@infradead.org>
Cc:     akpm@linux-foundation.org, corbet@lwn.net, linux-mm@kvack.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        rick.p.edgecombe@intel.com
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On Sat, Jul 22, 2023 at 9:56=E2=80=AFPM Matthew Wilcox <willy@infradead.org=
> wrote:
>
> On Sat, Jul 22, 2023 at 11:15:06PM +0000, Pasha Tatashin wrote:
> >  static struct page_table_check *get_page_table_check(struct page_ext *=
page_ext)
> >  {
> > -     BUG_ON(!page_ext);
> > +     PAGE_TABLE_CHECK_WARN(!page_ext);
> > +
> >       return (void *)(page_ext) + page_table_check_ops.offset;
> >  }
>
> [...]
>
> > @@ -137,15 +144,15 @@ void __page_table_check_zero(struct page *page, u=
nsigned int order)
> >       struct page_ext *page_ext;
> >       unsigned long i;
> >
> > -     BUG_ON(PageSlab(page));
> > +     PAGE_TABLE_CHECK_WARN(PageSlab(page));
> >
> >       page_ext =3D page_ext_get(page);
> > -     BUG_ON(!page_ext);
> > +     PAGE_TABLE_CHECK_WARN(!page_ext);
> >       for (i =3D 0; i < (1ul << order); i++) {
> >               struct page_table_check *ptc =3D get_page_table_check(pag=
e_ext);
>
> Seems like we're going to warn about !page_ext twice?  Or more than
> twice -- once per tail page?
>
> But then we'll crash because page_ext was NULL and offset was small?

Good catch, page_ext should not be NULL, yet I do not want to add
BUG_ON, let me fix this by warning and gracefully returning if
page_ext is NULL

Pasha