From 4231aba000f5a4583dd9f67057aadb68c3eca99d Mon Sep 17 00:00:00 2001
From: Nicholas Piggin <npiggin@gmail.com>
Date: Fri, 27 Jul 2018 21:48:17 +1000
Subject: powerpc/64s: Fix page table fragment refcount race vs speculative
 references

The page table fragment allocator uses the main page refcount racily
with respect to speculative references. A customer observed a BUG due
to page table page refcount underflow in the fragment allocator. This
can be caused by the fragment allocator set_page_count stomping on a
speculative reference, and then the speculative failure handler
decrements the new reference, and the underflow eventually pops when
the page tables are freed.

Fix this by using a dedicated field in the struct page for the page
table fragment allocator.

Fixes: 5c1f6ee9a31c ("powerpc: Reduce PTE table memory wastage")
Cc: stable@vger.kernel.org # v3.10+
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
 include/linux/mm_types.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

(limited to 'include/linux')

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 99ce070e7dcb..22651e124071 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -139,7 +139,10 @@ struct page {
 			unsigned long _pt_pad_1;	/* compound_head */
 			pgtable_t pmd_huge_pte; /* protected by page->ptl */
 			unsigned long _pt_pad_2;	/* mapping */
-			struct mm_struct *pt_mm;	/* x86 pgds only */
+			union {
+				struct mm_struct *pt_mm; /* x86 pgds only */
+				atomic_t pt_frag_refcount; /* powerpc */
+			};
 #if ALLOC_SPLIT_PTLOCKS
 			spinlock_t *ptl;
 #else
-- 
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