[PATCH 1/2] powerpc/mm/radix: Properly clear process table entry

On radix, the process table entry we want to clear when
destroying a context is entry 0, not entry 1

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
---
 arch/powerpc/mm/mmu_context_book3s64.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/mm/mmu_context_book3s64.c b/arch/powerpc/mm/mmu_context_book3s64.c
index 79d8c8a..9404b5e 100644
--- a/arch/powerpc/mm/mmu_context_book3s64.c
+++ b/arch/powerpc/mm/mmu_context_book3s64.c

@@ -226,9 +226,15 @@ void destroy_context(struct mm_struct *mm)
 	mm->context.cop_lockp = NULL;
 #endif /* CONFIG_PPC_ICSWX */
 
-	if (radix_enabled())
-		process_tb[mm->context.id].prtb1 = 0;
-	else
+	if (radix_enabled()) {
+		/*
+		 * Radix doesn't have a valid bit in the process table
+		 * entries. However we know that at least P9 implementation
+		 * will avoid caching an entry with an invalid RTS field,
+		 * and 0 is invalid. So this will do.
+		 */
+		process_tb[mm->context.id].prtb0 = 0;
+	} else
 		subpage_prot_free(mm);
 	destroy_pagetable_page(mm);
 	__destroy_context(mm->context.id);

Benjamin Herrenschmidt [off-list ref] writes:

On radix, the process table entry we want to clear when
destroying a context is entry 0, not entry 1

.. but has no real world consequence? Or causes the system to instantly
catch fire?


I'll tag it:

Fixes: 7e381c0ff618 ("powerpc/mm/radix: Add mmu context handling callback for radix")

cheers

On Mon, 2017-07-10 at 13:51 +1000, Michael Ellerman wrote:

Benjamin Herrenschmidt [off-list ref] writes:

quoted

On radix, the process table entry we want to clear when
destroying a context is entry 0, not entry 1

.. but has no real world consequence? Or causes the system to instantly
catch fire?

It has no *immediate* consequence on P9 , but it can cause other bugs
to become worse (such as the KVM issue I'm chasing).

I'll tag it:

Fixes: 7e381c0ff618 ("powerpc/mm/radix: Add mmu context handling callback for radix")

cheers

quoted

diff --git a/arch/powerpc/mm/mmu_context_book3s64.c b/arch/powerpc/mm/mmu_context_book3s64.c
index 79d8c8a..9404b5e 100644
--- a/arch/powerpc/mm/mmu_context_book3s64.c
+++ b/arch/powerpc/mm/mmu_context_book3s64.c

@@ -226,9 +226,15 @@ void destroy_context(struct mm_struct *mm)
 	mm->context.cop_lockp = NULL;
 #endif /* CONFIG_PPC_ICSWX */
 
-	if (radix_enabled())
-		process_tb[mm->context.id].prtb1 = 0;
-	else
+	if (radix_enabled()) {
+		/*
+		 * Radix doesn't have a valid bit in the process table
+		 * entries. However we know that at least P9 implementation
+		 * will avoid caching an entry with an invalid RTS field,
+		 * and 0 is invalid. So this will do.
+		 */
+		process_tb[mm->context.id].prtb0 = 0;
+	} else
 		subpage_prot_free(mm);
 	destroy_pagetable_page(mm);
 	__destroy_context(mm->context.id);

On Sat, 08 Jul 2017 07:45:32 -0500
Benjamin Herrenschmidt [off-list ref] wrote:

On radix, the process table entry we want to clear when
destroying a context is entry 0, not entry 1

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

This looks obviously correct to me.

Benjamin Herrenschmidt [off-list ref] writes:

On radix, the process table entry we want to clear when
destroying a context is entry 0, not entry 1

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

Reviewed-by: Aneesh Kumar K.V <redacted>

Benjamin Herrenschmidt [off-list ref] writes:

On Mon, 2017-07-10 at 13:51 +1000, Michael Ellerman wrote:

quoted

Benjamin Herrenschmidt [off-list ref] writes:

quoted

On radix, the process table entry we want to clear when
destroying a context is entry 0, not entry 1

.. but has no real world consequence? Or causes the system to instantly
catch fire?

It has no *immediate* consequence on P9 , but it can cause other bugs
to become worse (such as the KVM issue I'm chasing).

OK, I'll flag it for stable then to be safe.

cheers

On Sat, 2017-07-08 at 12:45:32 UTC, Benjamin Herrenschmidt wrote:

On radix, the process table entry we want to clear when
destroying a context is entry 0, not entry 1

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: Aneesh Kumar K.V <redacted>

Applied to powerpc fixes, thanks.

https://git.kernel.org/powerpc/c/c6bb0b8d426a8cf865ca9c8a532cc3

cheers