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

[Ingo Molnar]

* Ingo Molnar <mingo@xxxxxxxxxx> wrote:

> numa/core profile:
> 
>     95.66%  perf-1201.map     [.] 0x00007fe4ad1c8fc7                 
>      1.70%  libjvm.so         [.] 0x0000000000381581                 
>      0.59%  [vdso]            [.] 0x0000000000000607                 
>      0.19%  [kernel]          [k] do_raw_spin_lock                   
>      0.11%  [kernel]          [k] generic_smp_call_function_interrupt
>      0.11%  [kernel]          [k] timekeeping_get_ns.constprop.7     
>      0.08%  [kernel]          [k] ktime_get                          
>      0.06%  [kernel]          [k] get_cycles                         
>      0.05%  [kernel]          [k] __native_flush_tlb                 
>      0.05%  [kernel]          [k] rep_nop                            
>      0.04%  perf              [.] add_hist_entry.isra.9              
>      0.04%  [kernel]          [k] rcu_check_callbacks                
>      0.04%  [kernel]          [k] ktime_get_update_offsets           
>      0.04%  libc-2.15.so      [.] __strcmp_sse2                      
> 
> No page fault overhead (see the page fault rate further below) 
> - the NUMA scanning overhead shows up only through some mild 
> TLB flush activity (which I'll fix btw).

The patch attached below should get rid of that mild TLB 
flushing activity as well.

Thanks,

	Ingo

--------------------------->
Subject: x86/mm: Don't flush the TLB on #WP pmd fixups
From: Ingo Molnar <mingo@xxxxxxxxxx>
Date: Tue Nov 20 14:46:34 CET 2012

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@xxxxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/mm/pgtable.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

Index: linux/arch/x86/mm/pgtable.c
===================================================================
--- linux.orig/arch/x86/mm/pgtable.c
+++ linux/arch/x86/mm/pgtable.c
@@ -334,7 +334,12 @@ int pmdp_set_access_flags(struct vm_area
 	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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