linux-kernel - [PATCH] x86/mm: Change barriers before TLB flushes to smp_mb__after

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Date:	Fri, 27 May 2016 20:16:51 -0700
From:	Nadav Amit <namit@...are.com>
To:	x86@...nel.org, linux-kernel@...r.kernel.org
Cc:	nadav.amit@...il.com, Nadav Amit <namit@...are.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Andy Lutomirski <luto@...nel.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Michal Hocko <mhocko@...e.com>,
	Vladimir Davydov <vdavydov@...tuozzo.com>,
	Jerome Marchand <jmarchan@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>,
	Minchan Kim <minchan@...nel.org>,
	linux-mm@...ck.org (open list:MEMORY MANAGEMENT)
Subject: [PATCH] x86/mm: Change barriers before TLB flushes to smp_mb__after_atomic

When (current->active_mm != mm), flush_tlb_page() does not perform a
memory barrier. In practice, this memory barrier is not needed since in
the existing call-sites the PTE is modified using atomic-operations.
This patch therefore modifies the existing smp_mb in flush_tlb_page to
smp_mb__after_atomic and adds the missing one, while documenting the new
assumption of flush_tlb_page.

In addition smp_mb__after_atomic is also added to
set_tlb_ubc_flush_pending, since it makes a similar implicit assumption
and omits the memory barrier.

Signed-off-by: Nadav Amit <namit@...are.com>
---
 arch/x86/mm/tlb.c | 9 ++++++++-
 mm/rmap.c         | 3 +++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index fe9b9f7..2534333 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -242,6 +242,10 @@ out:
 	preempt_enable();
 }
 
+/*
+ * Calls to flush_tlb_page must be preceded by atomic PTE change or
+ * explicit memory-barrier.
+ */
 void flush_tlb_page(struct vm_area_struct *vma, unsigned long start)
 {
 	struct mm_struct *mm = vma->vm_mm;
@@ -259,8 +263,11 @@ void flush_tlb_page(struct vm_area_struct *vma, unsigned long start)
 			leave_mm(smp_processor_id());
 
 			/* Synchronize with switch_mm. */
-			smp_mb();
+			smp_mb__after_atomic();
 		}
+	} else {
+		/* Synchronize with switch_mm. */
+		smp_mb__after_atomic();
 	}
 
 	if (cpumask_any_but(mm_cpumask(mm), smp_processor_id()) < nr_cpu_ids)
diff --git a/mm/rmap.c b/mm/rmap.c
index 307b555..60ab0fe 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -613,6 +613,9 @@ static void set_tlb_ubc_flush_pending(struct mm_struct *mm,
 {
 	struct tlbflush_unmap_batch *tlb_ubc = &current->tlb_ubc;
 
+	/* Synchronize with switch_mm. */
+	smp_mb__after_atomic();
+
 	cpumask_or(&tlb_ubc->cpumask, &tlb_ubc->cpumask, mm_cpumask(mm));
 	tlb_ubc->flush_required = true;
 
-- 
2.7.4