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Date:   Wed, 10 May 2023 19:57:46 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Geert Uytterhoeven <geert@linux-m68k.org>
cc:     Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@kernel.org>,
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        Matthew Wilcox <willy@infradead.org>,
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Subject: Re: [PATCH 05/23] m68k: allow pte_offset_map[_lock]() to fail
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On Wed, 10 May 2023, Geert Uytterhoeven wrote:

> Hi Hugh,
>=20
> Thanks for your patch!

And thank you for looking so quickly, Geert.

>=20
> On Wed, May 10, 2023 at 6:48=E2=80=AFAM Hugh Dickins <hughd@google.com> w=
rote:
> > In rare transient cases, not yet made possible, pte_offset_map() and
> > pte_offset_map_lock() may not find a page table: handle appropriately.
> >
> > Restructure cf_tlb_miss() with a pte_unmap() (previously omitted)
> > at label out, followed by one local_irq_restore() for all.
>=20
> That's a bug fix, which should be a separate patch?

No, that's not a bug fix for the current tree, since m68k does not
offer CONFIG_HIGHPTE, so pte_unmap() is never anything but a no-op
for m68k (see include/linux/pgtable.h).

But I want to change pte_unmap() to do something even without
CONFIG_HIGHPTE, so have to fix up any such previously harmless
omissions in this series first.

>=20
> >
> > Signed-off-by: Hugh Dickins <hughd@google.com>
>=20
>=20
> > --- a/arch/m68k/include/asm/mmu_context.h
> > +++ b/arch/m68k/include/asm/mmu_context.h
> > @@ -99,7 +99,7 @@ static inline void load_ksp_mmu(struct task_struct *t=
ask)
> >         p4d_t *p4d;
> >         pud_t *pud;
> >         pmd_t *pmd;
> > -       pte_t *pte;
> > +       pte_t *pte =3D NULL;
> >         unsigned long mmuar;
> >
> >         local_irq_save(flags);
> > @@ -139,7 +139,7 @@ static inline void load_ksp_mmu(struct task_struct =
*task)
> >
> >         pte =3D (mmuar >=3D PAGE_OFFSET) ? pte_offset_kernel(pmd, mmuar=
)
> >                                      : pte_offset_map(pmd, mmuar);
> > -       if (pte_none(*pte) || !pte_present(*pte))
> > +       if (!pte || pte_none(*pte) || !pte_present(*pte))
> >                 goto bug;
>=20
> If the absence of a pte is to become a non-abnormal case, it should
> probably jump to "end" instead, to avoid spamming the kernel log.

I don't think so (but of course it's hard for you to tell, without
seeing all completed series of series).  If pmd_none(*pmd) can safely
goto bug just above, and pte_none(*pte) goto bug here, well, the !pte
case is going to be stranger than either of those.

My understanding of this function, load_ksp_mmu(), is that it's dealing
at context switch with a part of userspace which very much needs to be
present: whatever keeps that from being swapped out or migrated at
present, will be sure to keep the !pte case away - we cannot steal its
page table just at random (and a THP on m68k would be surprising too).

Though there is one case I can think of which will cause !pte here,
and so goto bug: if the pmd entry has got corrupted, and counts as
pmd_bad(), which will be tested (and cleared) in pte_offset_map().
But it is okay to report a bug in that case.

I can certainly change this to goto end instead if you still prefer,
no problem; but I'd rather keep it as is, if only for me to be proved
wrong by you actually seeing spam there.

>=20
> >
> >         set_pte(pte, pte_mkyoung(*pte));
> > @@ -161,6 +161,8 @@ static inline void load_ksp_mmu(struct task_struct =
*task)
> >  bug:
> >         pr_info("ksp load failed: mm=3D0x%p ksp=3D0x08%lx\n", mm, mmuar=
);
> >  end:
> > +       if (pte && mmuar < PAGE_OFFSET)
> > +               pte_unmap(pte);
>=20
> Is this also a bugfix, not mentioned in the patch description?

I'm not sure whether you're referring to the pte_unmap() which we
already discussed above, or you're seeing something else in addition;
but I don't think there's a bugfix here, just a rearrangement because
we now want lots of cases to do the pte_unmap() and local_irq_restore().

Hugh

>=20
> >         local_irq_restore(flags);
> >  }
> >
>=20
> Gr{oetje,eeting}s,
>=20
>                         Geert
>=20
> --=20
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m6=
8k.org
>=20
> 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
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