mm: Only flush TLBs if a transhuge PMD is modified for NUMA pte scanning

NUMA PTE scanning is expensive both in terms of the scanning itself and
the TLB flush if there are any updates. The TLB flush is avoided if no
PTEs are updated but there is a bug where transhuge PMDs are considered
to be updated even if they were already pmd_numa. This patch addresses
the problem and TLB flushes should be reduced.

Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1381141781-10992-12-git-send-email-mgorman@suse.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 10 insertions, 4 deletions

diff --git a/mm/mprotect.c b/mm/mprotect.c
index 7bdbd4b0f6d9..2da33dca6134 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -144,10 +144,16 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
                 if (pmd_trans_huge(*pmd)) {
                         if (next - addr != HPAGE_PMD_SIZE)
                                 split_huge_page_pmd(vma, addr, pmd);
-                        else if (change_huge_pmd(vma, pmd, addr, newprot,
-                                                 prot_numa)) {
-                                pages++;
-                                continue;
+                        else {
+                                int nr_ptes = change_huge_pmd(vma, pmd, addr,
+                                                newprot, prot_numa);
+
+                                if (nr_ptes) {
+                                        if (nr_ptes == HPAGE_PMD_NR)
+                                                pages++;
+
+                                        continue;
+                                }
                         }
                         /* fall through */
                 }