Re: [PATCH RFC] mm: arm64: advance pte for contpte_ptep_set_access_flags

Barry Song
2024-08-31T10:06:55Z

On Sat, Aug 31, 2024 at 9:54 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 31.08.24 10:35, Barry Song wrote:
> > From: Barry Song <v-songbaohua@oppo.com>
> >
> > Hi Ryan, David,
> > it seems contpte_ptep_set_access_flags() has never advanced
> > pte pfn, and it is setting all entries' pfn to the first
> > subpage. But I feel quite strange we never have a bug reported.
> > Am I missing something?
> >
> > Fixes: 4602e5757bcc ("arm64/mm: wire up PTE_CONT for user mappings")
> > Cc: Ard Biesheuvel <ardb@kernel.org>
> > Cc: John Hubbard <jhubbard@nvidia.com>
> > Cc: Mark Rutland <mark.rutland@arm.com>
> > Cc: Catalin Marinas <catalin.marinas@arm.com>
> > Cc: David Hildenbrand <david@redhat.com>
> > Cc: Will Deacon <will@kernel.org>
> > Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> > ---
> > arch/arm64/mm/contpte.c | 4 +++-
> > 1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
> > index a3edced29ac1..10dcd2641184 100644
> > --- a/arch/arm64/mm/contpte.c
> > +++ b/arch/arm64/mm/contpte.c
> > @@ -421,8 +421,10 @@ int contpte_ptep_set_access_flags(struct vm_area_struct *vma,
> > ptep = contpte_align_down(ptep);
> > start_addr = addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
> >
> > - for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE)
> > + for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE) {
> > __ptep_set_access_flags(vma, addr, ptep, entry, 0);
> > + entry = pte_advance_pfn(entry, 1);
> > + }
> >
> > if (dirty)
> > __flush_tlb_range(vma, start_addr, addr,
>
> Taking a closer look at __ptep_set_access_flags(), there is:
>
> /* only preserve the access flags and write permission *
> pte_val(entry) &= PTE_RDONLY | PTE_AF | PTE_WRITE | PTE_DIRTY;
>
> So it looks like it doesn't need the PFN?

right.

>
>
> OTOH, there is the initial:
>
>
> if (pte_same(pte, entry))
> return 0;
>
> check that might accelerate things.
>
> So unless I am missing something, this works as expected? (and if the
> pte_same() would frequently be taken with your change would be worthwile
> to optimize)

Right. From page 1 to page (nr_pages - 1), we consistently get FALSE
for pte_same().
This seems quite strange. I think we might need to "fix" it, at least
for the sake of code
semantics. on the other hand, if pte_same() is not important, it
should be dropped.

Hi Ryan,
what is your take on this?

>
> --
> Cheers,
>
> David / dhildenb
>

Thanks
Barry