[PATCH 05/31] x86/mm: Reduce tlb flushes from ptep_set_access_flags()

[Peter Zijlstra]

From: Rik van Riel <riel@xxxxxxxxxx>

If ptep_set_access_flags() is invoked to upgrade access permissions
on a PTE, there is no security or data integrity reason to do a
remote TLB flush.

Lazily letting another CPU incur a spurious page fault occasionally
is (much!) cheaper than aggressively flushing everybody else's TLB.

Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/mm/pgtable.c |   17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

Index: tip/arch/x86/mm/pgtable.c
===================================================================
--- tip.orig/arch/x86/mm/pgtable.c
+++ tip/arch/x86/mm/pgtable.c
@@ -306,11 +306,26 @@ int ptep_set_access_flags(struct vm_area
 			  pte_t entry, int dirty)
 {
 	int changed = !pte_same(*ptep, entry);
+	/*
+	 * If the page used to be inaccessible (_PAGE_PROTNONE), or
+	 * this call upgrades the access permissions on the same page,
+	 * it is safe to skip the remote TLB flush.
+	 */
+	bool flush_remote = false;
+	if (!pte_accessible(*ptep))
+		flush_remote = false;
+	else if (pte_pfn(*ptep) != pte_pfn(entry) ||
+			(pte_write(*ptep) && !pte_write(entry)) ||
+			(pte_exec(*ptep) && !pte_exec(entry)))
+		flush_remote = true;
 
 	if (changed && dirty) {
 		*ptep = entry;
 		pte_update_defer(vma->vm_mm, address, ptep);
-		flush_tlb_page(vma, address);
+		if (flush_remote)
+			flush_tlb_page(vma, address);
+		else
+			__flush_tlb_one(address);
 	}
 
 	return changed;


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