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[PATCH v2] arm64: contpte: fix set_access_flags() no-op check for SMMU/ATS faults To: Piotr Jaroszynski , Catalin Marinas , Will Deacon , Ard Biesheuvel , Ryan Roberts , Mark Rutland , Andrew Morton Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, John Hubbard , Zi Yan , Breno Leitao , stable@vger.kernel.org, Alistair Popple , James Houghton , Jason Gunthorpe References: <20260305-contpte-fault-loop-v2-1-0216f0026d7f@nvidia.com> Content-Language: en-US From: Balbir Singh In-Reply-To: <20260305-contpte-fault-loop-v2-1-0216f0026d7f@nvidia.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: BY5PR20CA0015.namprd20.prod.outlook.com (2603:10b6:a03:1f4::28) To PH8PR12MB7277.namprd12.prod.outlook.com (2603:10b6:510:223::13) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH8PR12MB7277:EE_|SN7PR12MB7203:EE_ X-MS-Office365-Filtering-Correlation-Id: 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can make the > target appear already-dirty. When the gathered value matches entry, the > function returns 0 even though the target sub-PTE still has PTE_RDONLY > set in hardware. > > For a CPU with FEAT_HAFDBS this gathered view is fine, since hardware may > set AF/dirty on any sub-PTE and CPU TLB behavior is effectively gathered > across the CONT range. But page-table walkers that evaluate each > descriptor individually (e.g. a CPU without DBM support, or an SMMU > without HTTU, or with HA/HD disabled in CD.TCR) can keep faulting on the > unchanged target sub-PTE, causing an infinite fault loop. > > Gathering can therefore cause false no-ops when only a sibling has been > updated: > - write faults: target still has PTE_RDONLY (needs PTE_RDONLY cleared) > - read faults: target still lacks PTE_AF > > Fix by checking each sub-PTE against the requested AF/dirty/write state > (the same bits consumed by __ptep_set_access_flags()), using raw > per-PTE values rather than the gathered ptep_get() view, before > returning no-op. Keep using the raw target PTE for the write-bit unfold > decision. > > Per Arm ARM (DDI 0487) D8.7.1 ("The Contiguous bit"), any sub-PTE in a CONT > range may become the effective cached translation and software must > maintain consistent attributes across the range. > > Fixes: 4602e5757bcc ("arm64/mm: wire up PTE_CONT for user mappings") > Cc: Ryan Roberts > Cc: Catalin Marinas > Cc: Will Deacon > Cc: Jason Gunthorpe > Cc: John Hubbard > Cc: Zi Yan > Cc: Breno Leitao > Cc: stable@vger.kernel.org > Reviewed-by: Alistair Popple > Reviewed-by: James Houghton > Reviewed-by: Ryan Roberts > Reviewed-by: Catalin Marinas > Tested-by: Breno Leitao > Signed-off-by: Piotr Jaroszynski > --- > Changes in v2: > - Clarify commit message/comments: issue affects per-descriptor walkers > (CPU without DBM support, or SMMU without HTTU / with HA/HD disabled). > - Clarify sub-PTE comparison semantics: use raw per-PTE values and match > bits consumed by __ptep_set_access_flags() (AF, DIRTY, write permission). > - Add Reviewed-by/Tested-by trailers from the v1 thread. > --- > arch/arm64/mm/contpte.c | 53 +++++++++++++++++++++++++++++++++++++++++++++---- > 1 file changed, 49 insertions(+), 4 deletions(-) > > diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c > index b929a455103f..1519d090d5ea 100644 > --- a/arch/arm64/mm/contpte.c > +++ b/arch/arm64/mm/contpte.c > @@ -599,6 +599,27 @@ void contpte_clear_young_dirty_ptes(struct vm_area_struct *vma, > } > EXPORT_SYMBOL_GPL(contpte_clear_young_dirty_ptes); > > +static bool contpte_all_subptes_match_access_flags(pte_t *ptep, pte_t entry) > +{ > + pte_t *cont_ptep = contpte_align_down(ptep); > + /* > + * PFNs differ per sub-PTE. Match only bits consumed by > + * __ptep_set_access_flags(): AF, DIRTY and write permission. > + */ > + const pteval_t cmp_mask = PTE_RDONLY | PTE_AF | PTE_WRITE | PTE_DIRTY; > + pteval_t entry_cmp = pte_val(entry) & cmp_mask; > + int i; > + > + for (i = 0; i < CONT_PTES; i++) { > + pteval_t pte_cmp = pte_val(__ptep_get(cont_ptep + i)) & cmp_mask; > + > + if (pte_cmp != entry_cmp) > + return false; > + } > + > + return true; > +} > + > int contpte_ptep_set_access_flags(struct vm_area_struct *vma, > unsigned long addr, pte_t *ptep, > pte_t entry, int dirty) > @@ -608,13 +629,37 @@ int contpte_ptep_set_access_flags(struct vm_area_struct *vma, > int i; > > /* > - * Gather the access/dirty bits for the contiguous range. If nothing has > - * changed, its a noop. > + * Check whether all sub-PTEs in the CONT block already match the > + * requested access flags/write permission, using raw per-PTE values > + * rather than the gathered ptep_get() view. > + * > + * __ptep_set_access_flags() can update AF, dirty and write > + * permission, but only to make the mapping more permissive. > + * > + * ptep_get() gathers AF/dirty state across the whole CONT block, > + * which is correct for a CPU with FEAT_HAFDBS. But page-table > + * walkers that evaluate each descriptor individually (e.g. a CPU > + * without DBM support, or an SMMU without HTTU, or with HA/HD > + * disabled in CD.TCR) can keep faulting on the target sub-PTE if > + * only a sibling has been updated. Gathering can therefore cause > + * false no-ops when only a sibling has been updated: > + * - write faults: target still has PTE_RDONLY (needs PTE_RDONLY cleared) > + * - read faults: target still lacks PTE_AF > + * > + * Per Arm ARM (DDI 0487) D8.7.1, any sub-PTE in a CONT range may > + * become the effective cached translation, so all entries must have > + * consistent attributes. Check the full CONT block before returning > + * no-op, and when any sub-PTE mismatches, proceed to update the whole > + * range. > */ > - orig_pte = pte_mknoncont(ptep_get(ptep)); > - if (pte_val(orig_pte) == pte_val(entry)) > + if (contpte_all_subptes_match_access_flags(ptep, entry)) > return 0; > > + /* > + * Use raw target pte (not gathered) for write-bit unfold decision. > + */ > + orig_pte = pte_mknoncont(__ptep_get(ptep)); > + > /* > * We can fix up access/dirty bits without having to unfold the contig > * range. But if the write bit is changing, we must unfold. > Looks good Acked-by: Balbir Singh Balbir