linux-kernel - [tip:x86/mm] x86/mm: Don't flush the TLB on #WP pmd fixups

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Date:	Thu, 22 Nov 2012 15:22:39 -0800
From:	tip-bot for Ingo Molnar <mingo@...nel.org>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...nel.org,
	a.p.zijlstra@...llo.nl, torvalds@...ux-foundation.org,
	pjt@...gle.com, hannes@...xchg.org, hughd@...gle.com, cl@...ux.com,
	riel@...hat.com, akpm@...ux-foundation.org, aarcange@...hat.com,
	Lee.Schermerhorn@...com, mgorman@...e.de, tglx@...utronix.de
Subject: [tip:x86/mm] x86/mm: Don't flush the TLB on #WP pmd fixups

Commit-ID:  5e4bf1a55da976a5ed60901bb8801f1024ef9774
Gitweb:     http://git.kernel.org/tip/5e4bf1a55da976a5ed60901bb8801f1024ef9774
Author:     Ingo Molnar <mingo@...nel.org>
AuthorDate: Tue, 20 Nov 2012 13:02:51 +0100
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Thu, 22 Nov 2012 21:52:06 +0100

x86/mm: Don't flush the TLB on #WP pmd fixups

If we have a write protection #PF and fix up the pmd then the
hugetlb code [the only user of pmdp_set_access_flags], in its
do_huge_pmd_wp_page() page fault resolution function calls
pmdp_set_access_flags() to mark the pmd permissive again,
and flushes the TLB.

This TLB flush is unnecessary: a flush on #PF is guaranteed on
most (all?) x86 CPUs, and even in the worst-case we'll generate
a spurious fault.

So remove it.

Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc: Paul Turner <pjt@...gle.com>
Cc: Lee Schermerhorn <Lee.Schermerhorn@...com>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: Rik van Riel <riel@...hat.com>
Cc: Johannes Weiner <hannes@...xchg.org>
Cc: Christoph Lameter <cl@...ux.com>
Cc: Mel Gorman <mgorman@...e.de>
Cc: Hugh Dickins <hughd@...gle.com>
Link: http://lkml.kernel.org/r/20121120120251.GA15742@gmail.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/mm/pgtable.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index 8573b83..8a828d7 100644
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -328,7 +328,12 @@ int pmdp_set_access_flags(struct vm_area_struct *vma,
 	if (changed && dirty) {
 		*pmdp = entry;
 		pmd_update_defer(vma->vm_mm, address, pmdp);
-		flush_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
+		/*
+		 * We had a write-protection fault here and changed the pmd
+		 * to to more permissive. No need to flush the TLB for that,
+		 * #PF is architecturally guaranteed to do that and in the
+		 * worst-case we'll generate a spurious fault.
+		 */
 	}
 
 	return changed;
--
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