On 04/09/2024 16:13, Will Deacon wrote:

(Adding Ryan, since you're asking him a question!)

Thanks, Will!

Afraid I don't do a good job of monitoring the list; I'm guessing there are
automated ways to filter for mentions of my name so I catch this sort of thing
in future?

On Sat, Aug 31, 2024 at 10:06:40PM +1200, Barry Song wrote:

quoted

On Sat, Aug 31, 2024 at 9:54 PM David Hildenbrand [off-list ref] wrote:

quoted

On 31.08.24 10:35, Barry Song wrote:

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From: Barry Song <redacted>

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 <redacted>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Barry Song <redacted>
---
  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?

Correct, I don't believe there is a bug here; __ptep_set_access_flags() only
consumes the access flags from entry.

quoted

right.

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OTOH, there is the initial:


if (pte_same(pte, entry))
        return 0;

check that might accelerate things.

There is an equivalent check in contpte_ptep_set_access_flags() which is
checking for the whole contpte block and returning early if so. So I don't think
there is a problem here either.

quoted

quoted

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?

The code is correct and working as intended, AFAICT. But I accept that this is
not exactly obvious. I'd be happy to Rb your proposed change if you feel it
clarifies things.

Thanks,
Ryan

quoted

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--
Cheers,

David / dhildenb

Thanks
Barry