[SRU][Bionic][v2][PATCH 2/2] powerpc/mm/radix: Only need the Nest MMU workaround for R -> RW transition

[Joseph Salisbury]

From: "Aneesh Kumar K.V" <aneesh.kumar at linux.ibm.com>

BugLink: https://bugs.launchpad.net/bugs/1792195

The Nest MMU workaround is only needed for RW upgrades. Avoid doing
that for other PTE updates.

We also avoid clearing the PTE while marking it invalid. This is
because other page table walkers will find this PTE none and can
result in unexpected behaviour due to that. Instead we clear
_PAGE_PRESENT and set the software PTE bit _PAGE_INVALID.
pte_present() is already updated to check for both bits. This makes
sure page table walkers will find the PTE present and things like
pte_pfn(pte) returns the right value.

Based on an original patch from Benjamin Herrenschmidt <benh at kernel.crashing.org>

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar at linux.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin at gmail.com>
Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
(backported from commit f08d08f3db55452d31ba4a37c702da6245876b96
[jsalisbury: Patch was not expcting cpu_has_feature() check, so
merged it in.])
Signed-off-by: Joseph Salisbury <joseph.salisbury at canonical.com>
---
 arch/powerpc/mm/pgtable-radix.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/mm/pgtable-radix.c b/arch/powerpc/mm/pgtable-radix.c
index a778560..42e2b37 100644
--- a/arch/powerpc/mm/pgtable-radix.c
+++ b/arch/powerpc/mm/pgtable-radix.c
@@ -1025,21 +1025,23 @@ void radix__ptep_set_access_flags(struct vm_area_struct *vma, pte_t *ptep,
 	struct mm_struct *mm = vma->vm_mm;
 	unsigned long set = pte_val(entry) & (_PAGE_DIRTY | _PAGE_ACCESSED |
 					      _PAGE_RW | _PAGE_EXEC);
+
+	unsigned long change = pte_val(entry) ^ pte_val(*ptep);
 	/*
 	 * To avoid NMMU hang while relaxing access, we need mark
 	 * the pte invalid in between.
 	 */
 	if (cpu_has_feature(CPU_FTR_POWER9_DD1) ||
-	    atomic_read(&mm->context.copros) > 0) {
+	    ((change & _PAGE_RW) && atomic_read(&mm->context.copros) > 0)) {
 		unsigned long old_pte, new_pte;
 
-		old_pte = __radix_pte_update(ptep, ~0, 0);
+		old_pte = __radix_pte_update(ptep, _PAGE_PRESENT, _PAGE_INVALID);
 		/*
 		 * new value of pte
 		 */
 		new_pte = old_pte | set;
 		radix__flush_tlb_page_psize(mm, address, psize);
-		__radix_pte_update(ptep, 0, new_pte);
+		__radix_pte_update(ptep, _PAGE_INVALID, new_pte);
 	} else {
 		__radix_pte_update(ptep, 0, set);
 		radix__flush_tlb_page_psize(mm, address, psize);
-- 
2.7.4