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@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Turner <pjt@google.com>
Cc: Lee Schermerhorn <Lee.Schermerhorn@hp.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Link: http://lkml.kernel.org/r/20121120120251.GA15742@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index 8573b83..8a828d7 100644 (file)
--- 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;