From: Barry Song <v-songbaohua@xxxxxxxx> According to David and Ryan, there isn't a bug here, even though we don't advance the PTE entry, because __ptep_set_access_flags() only uses the access flags from the entry. However, we always check pte_same(pte, entry) using the first entry in __ptep_set_access_flags(). This means that the checks from 1 to nr - 1 are not comparing the same PTE indexes (thus, they always return false), which can be a bit confusing. To clarify the code, let's add some comments. Reviewed-by: Ryan Roberts <ryan.roberts@xxxxxxx> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx> Cc: Ard Biesheuvel <ardb@xxxxxxxxxx> Cc: John Hubbard <jhubbard@xxxxxxxxxx> Cc: Mark Rutland <mark.rutland@xxxxxxx> Cc: Catalin Marinas <catalin.marinas@xxxxxxx> Cc: David Hildenbrand <david@xxxxxxxxxx> Cc: Will Deacon <will@xxxxxxxxxx> --- -v2: * collect Ryan's rb, thanks! * doc why but not advance entry * refine changelog and subject -v1: https://lore.kernel.org/linux-mm/20240831083537.62111-1-21cnbao@xxxxxxxxx/ arch/arm64/mm/contpte.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c index a3edced29ac1..55107d27d3f8 100644 --- a/arch/arm64/mm/contpte.c +++ b/arch/arm64/mm/contpte.c @@ -421,6 +421,12 @@ 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); + /* + * We are not advancing entry because __ptep_set_access_flags() + * only consumes access flags from entry. And since we have checked + * for the whole contpte block and returned early, pte_same() + * within __ptep_set_access_flags() is likely false. + */ for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE) __ptep_set_access_flags(vma, addr, ptep, entry, 0); -- 2.39.3 (Apple Git-146)
