Re: [PATCH v1 4/4] Revert "arm64: Enforce BBM for huge IO/VMAP mappings"

[Chintan Pandya]

On 3/14/2018 4:16 PM, Marc Zyngier wrote:
> On 14/03/18 08:48, Chintan Pandya wrote:
> > This commit 15122ee2c515a ("arm64: Enforce BBM for huge
> > IO/VMAP mappings") is a temporary work-around until the
> > issues with CONFIG_HAVE_ARCH_HUGE_VMAP gets fixed.
> >
> > Revert this change as we have fixes for the issue.
> >
> > Signed-off-by: Chintan Pandya <cpandya@xxxxxxxxxxxxxx>
> > ---
> >   arch/arm64/mm/mmu.c | 8 --------
> >   1 file changed, 8 deletions(-)
> >
> > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > index c0df264..19116c6 100644
> > --- a/arch/arm64/mm/mmu.c
> > +++ b/arch/arm64/mm/mmu.c
> > @@ -935,10 +935,6 @@ int pud_set_huge(pud_t *pudp, phys_addr_t phys, pgprot_t prot)
> >   	pgprot_t sect_prot = __pgprot(PUD_TYPE_SECT |
> >   					pgprot_val(mk_sect_prot(prot)));
> >   
> > -	/* ioremap_page_range doesn't honour BBM */
> > -	if (pud_present(READ_ONCE(*pudp)))
> > -		return 0;
> > -
> >   	BUG_ON(phys & ~PUD_MASK);
> >   	if (pud_val(*pud) && !pud_huge(*pud))
> >   		free_page((unsigned long)__va(pud_val(*pud)));
> > @@ -952,10 +948,6 @@ int pmd_set_huge(pmd_t *pmdp, phys_addr_t phys, pgprot_t prot)
> >   	pgprot_t sect_prot = __pgprot(PMD_TYPE_SECT |
> >   					pgprot_val(mk_sect_prot(prot)));
> >   
> > -	/* ioremap_page_range doesn't honour BBM */
> > -	if (pmd_present(READ_ONCE(*pmdp)))
> > -		return 0;
> > -
> >   	BUG_ON(phys & ~PMD_MASK);
> >   	if (pmd_val(*pmd) && !pmd_huge(*pmd))
> >   		free_page((unsigned long)__va(pmd_val(*pmd)));
>
> But you're still not doing a BBM, right? What prevents a speculative
> access from using the (now freed) entry? The TLB invalidation you've
> introduce just narrows the window where bad things can happen.
Valid point. I will rework on these patches.

Thanks Marc.

> My gut feeling is that this series introduces more bugs than it fixes...
> If you're going to fix it, please fix it by correctly implementing BBM
> for huge mappings.
>
> Or am I missing something terribly obvious?
>
> 	M.

Chintan
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