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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Wed, 20 Apr 2022 16:44:08 +0200
Message-ID: <CAMuHMdWA7n588XUKy2yondnZpAw_afFBz8DHxH0Q3Tt53HHsHg@mail.gmail.com>
Subject: Re: Regression with v5.18-rc1 tag on STM32F7 and STM32H7 based boards
To:     Greg Ungerer <gerg@linux-m68k.org>
Cc:     Hugh Dickins <hughd@google.com>,
        Patrice CHOTARD <patrice.chotard@foss.st.com>,
        Arnd Bergmann <arnd@arndb.de>,
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        Borislav Petkov <bp@suse.de>,
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Hi Greg,

On Wed, Apr 20, 2022 at 3:53 PM Greg Ungerer <gerg@linux-m68k.org> wrote:
> On 16/4/22 10:58, Hugh Dickins wrote:
> > Just to wrap up this thread: the tentative arch/ patches below did not
> > go into 5.18-rc2, but 5.18-rc3 will contain
> > 1bdec44b1eee ("tmpfs: fix regressions from wider use of ZERO_PAGE")
> > which fixes a further issue, and deletes the line which gave you trouble.
> >
> > With arch/h8300 removed from linux-next, and arch/arm losing a page by
> > the patch below, I don't think it's worth my arguing for those changes.
> > I'd still prefer arch/m68k to expose its empty_zero_page in ZERO_PAGE(),
> > or else not allocate it; but I won't be pursuing this further.
>
> Thanks for pointing this out. It certainly does look wrong to me for
> the m68k nommu case. I am not aware of any existing issues caused by
> this - but there is no good reason not to fix it.
>
> So I propose this change. Build and run tested on my m68knommu targets.
>
> Regards
> Greg
>
>
>  From f809fb8fbca9e5e637b8fda380955bd799bb3926 Mon Sep 17 00:00:00 2001
> From: Greg Ungerer <gerg@linux-m68k.org>
> Date: Wed, 20 Apr 2022 23:27:47 +1000
> Subject: [PATCH] m68knommu: set ZERO_PAGE() allocated zeroed page
>
> The non-MMU m68k pagetable ZERO_PAGE() macro is being set to the
> somewhat non-sensical value of "virt_to_page(0)". The zeroth page
> is not in any way guaranteed to be a page full of "0". So the result
> is that ZERO_PAGE() will almost certainly contain random values.
>
> We already allocate a real "empty_zero_page" in the mm setup code shared
> between MMU m68k and non-MMU m68k. It is just not hooked up to the
> ZERO_PAGE() macro for the non-MMU m68k case.
>
> Fix ZERO_PAGE() to use the allocated "empty_zero_page" pointer.
>
> I am not aware of any specific issues caused by the old code.
>
> Link: https://lore.kernel.org/linux-m68k/2a462b23-5b8e-bbf4-ec7d-778434a3b9d7@google.com/T/#t
> Reported-by: Hugh Dickens <hughd@google.com>
> Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>
> ---
>   arch/m68k/include/asm/pgtable_no.h | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/m68k/include/asm/pgtable_no.h b/arch/m68k/include/asm/pgtable_no.h
> index 87151d67d91e..bce5ca56c388 100644
> --- a/arch/m68k/include/asm/pgtable_no.h
> +++ b/arch/m68k/include/asm/pgtable_no.h
> @@ -42,7 +42,8 @@ extern void paging_init(void);
>    * ZERO_PAGE is a global shared page that is always zero: used
>    * for zero-mapped memory areas etc..
>    */
> -#define ZERO_PAGE(vaddr)       (virt_to_page(0))
> +extern void *empty_zero_page;
> +#define ZERO_PAGE(vaddr)       (virt_to_page(empty_zero_page))
>
>   /*
>    * All 32bit addresses are effectively valid for vmalloc...

And after that (or combined with this?), this can be factored
out from arch/m68k/include/asm/pgtable_{mm,no}.h into
arch/m68k/include/asm/pgtable.h.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds