Subject: [PATCH v1 3/3] powerpc/8xx: document and enforce that split PT locks are not used

From: David Hildenbrand <[email-masked]>
Date: Fri, 26 Jul 2024 17:07:28 +0200

Right now, we cannot have split PT locks because 8xx does not support
SMP.

But for the sake of documentation *why* 8xx is fine regarding what
we documented in huge_pte_lockptr(), let's just add code to enforce it
at the same time as documenting it.

This should also make everybody who wants to copy from the 8xx approach of
supporting such unusual ways of mapping hugetlb folios aware that it gets
tricky once multiple page tables are involved.

Signed-off-by: David Hildenbrand <[email-masked]>
---
 arch/powerpc/mm/pgtable.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/powerpc/mm/pgtable.c b/arch/powerpc/mm/pgtable.c
index ab0656115424f..7316396e452d8 100644
--- a/arch/powerpc/mm/pgtable.c
+++ b/arch/powerpc/mm/pgtable.c
@@ -297,6 +297,12 @@ int huge_ptep_set_access_flags(struct vm_area_struct *vma,
 }
 
 #if defined(CONFIG_PPC_8xx)
+
+#if defined(CONFIG_SPLIT_PTE_PTLOCKS) || defined(CONFIG_SPLIT_PMD_PTLOCKS)
+/* We need the same lock to protect the PMD table and the two PTE tables. */
+#error "8M hugetlb folios are incompatible with split page table locks"
+#endif
+
 static void __set_huge_pte_at(pmd_t *pmd, pte_t *ptep, pte_basic_t val)
 {
 	pte_basic_t *entry = (pte_basic_t *)ptep;
-- 
2.45.2