mm: numa: Scan pages with elevated page_mapcount

Currently automatic NUMA balancing is unable to distinguish between false
shared versus private pages except by ignoring pages with an elevated
page_mapcount entirely. This avoids shared pages bouncing between the
nodes whose task is using them but that is ignored quite a lot of data.

This patch kicks away the training wheels in preparation for adding support
for identifying shared/private pages is now in place. The ordering is so
that the impact of the shared/private detection can be easily measured. Note
that the patch does not migrate shared, file-backed within vmas marked
VM_EXEC as these are generally shared library pages. Migrating such pages
is not beneficial as there is an expectation they are read-shared between
caches and iTLB and iCache pressure is generally low.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1381141781-10992-28-git-send-email-mgorman@suse.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 5 insertions, 7 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 914216733e0a..2a28c2c6c165 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1484,14 +1484,12 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
                         struct page *page = pmd_page(*pmd);
 
                         /*
-                         * Only check non-shared pages. Do not trap faults
-                         * against the zero page. The read-only data is likely
-                         * to be read-cached on the local CPU cache and it is
-                         * less useful to know about local vs remote hits on
-                         * the zero page.
+                         * Do not trap faults against the zero page. The
+                         * read-only data is likely to be read-cached on the
+                         * local CPU cache and it is less useful to know about
+                         * local vs remote hits on the zero page.
                          */
-                        if (page_mapcount(page) == 1 &&
-                            !is_huge_zero_page(page) &&
+                        if (!is_huge_zero_page(page) &&
                             !pmd_numa(*pmd)) {
                                 entry = pmdp_get_and_clear(mm, addr, pmd);
                                 entry = pmd_mknuma(entry);