[PATCH 3.16 070/305] powerpc/mm/hash64: Fix subpage protection with 4K HPTE config

[Ben Hutchings]

3.16.37-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Michael Ellerman <mpe@xxxxxxxxxxxxxx>

commit aac55d7573c5d46ed9a62818d5d3e69dd2060105 upstream.

With Linux page size of 64K and hardware only supporting 4K HPTE, if we
use subpage protection, we always fail for the subpage 0 as shown
below (using the selftest subpage_prot test):

  520175565:  (4520111850): Failed at 0x3fffad4b0000 (p=13,sp=0,w=0), want=fault, got=pass !
  4520890210: (4520826495): Failed at 0x3fffad5b0000 (p=29,sp=0,w=0), want=fault, got=pass !
  4521574251: (4521510536): Failed at 0x3fffad6b0000 (p=45,sp=0,w=0), want=fault, got=pass !
  4522258324: (4522194609): Failed at 0x3fffad7b0000 (p=61,sp=0,w=0), want=fault, got=pass !

This is because hash preload wrongly inserts the HPTE entry for subpage
0 without looking at the subpage protection information.

Fix it by teaching should_hash_preload() not to preload if we have
subpage protection configured for that range.

It appears this has been broken since it was introduced in 2008.

Fixes: fa28237cfcc5 ("[POWERPC] Provide a way to protect 4k subpages when using 64k pages")
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
[mpe: Rework into should_hash_preload() to avoid build fails w/SLICES=n]
Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 arch/powerpc/mm/hash_utils_64.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

--- a/arch/powerpc/mm/hash_utils_64.c
+++ b/arch/powerpc/mm/hash_utils_64.c
@@ -1199,8 +1199,16 @@ EXPORT_SYMBOL_GPL(hash_page);
 #ifdef CONFIG_PPC_MM_SLICES
 static bool should_hash_preload(struct mm_struct *mm, unsigned long ea)
 {
+	int psize = get_slice_psize(mm, ea);
+
 	/* We only prefault standard pages for now */
-	if (unlikely(get_slice_psize(mm, ea) != mm->context.user_psize))
+	if (unlikely(psize != mm->context.user_psize))
+		return false;
+
+	/*
+	 * Don't prefault if subpage protection is enabled for the EA.
+	 */
+	if (unlikely((psize == MMU_PAGE_4K) && subpage_protection(mm, ea)))
 		return false;
 
 	return true;