Subject: Re: [RFC PATCH 0/7] Replace _PAGE_NUMA with PAGE_NONE protections

From: Mel Gorman <[email-masked]>
Date: Tue, 18 Nov 2014 16:01:12 +0000

On Mon, Nov 17, 2014 at 01:56:19PM +0530, Aneesh Kumar K.V wrote:
> Mel Gorman <[email-masked]> writes:
> 
> > This is follow up from the "pipe/page fault oddness" thread.
> >
> > Automatic NUMA balancing depends on being able to protect PTEs to trap a
> > fault and gather reference locality information. Very broadly speaking it
> > would mark PTEs as not present and use another bit to distinguish between
> > NUMA hinting faults and other types of faults. It was universally loved
> > by everybody and caused no problems whatsoever. That last sentence might
> > be a lie.
> >
> > This series is very heavily based on patches from Linus and Aneesh to
> > replace the existing PTE/PMD NUMA helper functions with normal change
> > protections. I did alter and add parts of it but I consider them relatively
> > minor contributions. Note that the signed-offs here need addressing. I
> > couldn't use "From" or Signed-off-by from the original authors as the
> > patches had to be broken up and they were never signed off. I expect the
> > two people involved will just stick their signed-off-by on it.
> 
> 
> How about the additional change listed below for ppc64 ? One part of the
> patch is to make sure that we don't hit the WARN_ON in set_pte and set_pmd
> because we find the _PAGE_PRESENT bit set in case of numa fault. I
> ended up relaxing the check there.
> 

I folded the set_pte_at and set_pmd_at changes into the patch "mm: Convert
p[te|md]_numa users to p[te|md]_protnone_numa" with one change -- both
set_pte_at and set_pmd_at checks are under CONFIG_DEBUG_VM for consistency.

> Second part of the change is to add a WARN_ON to make sure we are
> not depending on DSISR_PROTFAULT for anything else. We ideally should not
> get a DSISR_PROTFAULT for PROT_NONE or NUMA fault. hash_page_mm do check
> whether the access is allowed by pte before inserting a pte into hash
> page table. Hence we will never find a PROT_NONE or PROT_NONE_NUMA ptes
> in hash page table. But it is good to run with VM_WARN_ON ?
> 

Due to the nature of the check and when they are hit, I converted it to
a WARN_ON_ONCE. Due to the exceptional circumstance the overhead should
be non-existant and shouldn't need to be hidden below VM_WARN_ON. I also
noted that with the patch the kernel  potentially no longer recovers
from this exceptional cirsumstance and instead falls through. To avoid
this, I preserved the "goto out_unlock".

Is this still ok?

---8<---
ppc64: Add paranoid warnings for unexpected DSISR_PROTFAULT

ppc64 should not be depending on DSISR_PROTFAULT and it's unexpected
if they are triggered. This patch adds warnings just in case they
are being accidentally depended upon.

Requires-signed-off-by: Aneesh Kumar K.V <[email-masked]>
Signed-off-by: Mel Gorman <[email-masked]>
---
 arch/powerpc/mm/copro_fault.c |  7 ++++++-
 arch/powerpc/mm/fault.c       | 20 +++++++++-----------
 2 files changed, 15 insertions(+), 12 deletions(-)

diff --git a/arch/powerpc/mm/copro_fault.c b/arch/powerpc/mm/copro_fault.c
index 5a236f0..46152aa 100644
--- a/arch/powerpc/mm/copro_fault.c
+++ b/arch/powerpc/mm/copro_fault.c
@@ -64,7 +64,12 @@ int copro_handle_mm_fault(struct mm_struct *mm, unsigned long ea,
 		if (!(vma->vm_flags & VM_WRITE))
 			goto out_unlock;
 	} else {
-		if (dsisr & DSISR_PROTFAULT)
+		/*
+		 * protfault should only happen due to us
+		 * mapping a region readonly temporarily. PROT_NONE
+		 * is also covered by the VMA check above.
+		 */
+		if (WARN_ON_ONCE(dsisr & DSISR_PROTFAULT))
 			goto out_unlock;
 		if (!(vma->vm_flags & (VM_READ | VM_EXEC)))
 			goto out_unlock;
diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index 5007497..9d6e0b3 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -396,17 +396,6 @@ good_area:
 #endif /* CONFIG_8xx */
 
 	if (is_exec) {
-#ifdef CONFIG_PPC_STD_MMU
-		/* Protection fault on exec go straight to failure on
-		 * Hash based MMUs as they either don't support per-page
-		 * execute permission, or if they do, it's handled already
-		 * at the hash level. This test would probably have to
-		 * be removed if we change the way this works to make hash
-		 * processors use the same I/D cache coherency mechanism
-		 * as embedded.
-		 */
-#endif /* CONFIG_PPC_STD_MMU */
-
 		/*
 		 * Allow execution from readable areas if the MMU does not
 		 * provide separate controls over reading and executing.
@@ -421,6 +410,14 @@ good_area:
 		    (cpu_has_feature(CPU_FTR_NOEXECUTE) ||
 		     !(vma->vm_flags & (VM_READ | VM_WRITE))))
 			goto bad_area;
+#ifdef CONFIG_PPC_STD_MMU
+		/*
+		 * protfault should only happen due to us
+		 * mapping a region readonly temporarily. PROT_NONE
+		 * is also covered by the VMA check above.
+		 */
+		WARN_ON_ONCE(error_code & DSISR_PROTFAULT);
+#endif /* CONFIG_PPC_STD_MMU */
 	/* a write */
 	} else if (is_write) {
 		if (!(vma->vm_flags & VM_WRITE))
@@ -430,6 +427,7 @@ good_area:
 	} else {
 		if (!(vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE)))
 			goto bad_area;
+		WARN_ON_ONCE(error_code & DSISR_PROTFAULT);
 	}
 
 	/*