writeback: fix dirtied pages accounting on sub-page writes

When dd in 512bytes, generic_perform_write() calls
balance_dirty_pages_ratelimited() 8 times for the same page, but
obviously the page is only dirtied once.

Fix it by accounting tsk->nr_dirtied and bdp_ratelimits at page dirty time.

Acked-by: Jan Kara <jack@suse.cz>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>

1 files changed, 5 insertions, 8 deletions

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 619c445fc03c..5d1ef5d8613a 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1258,8 +1258,6 @@ void balance_dirty_pages_ratelimited_nr(struct address_space *mapping,
         if (bdi->dirty_exceeded)
                 ratelimit = min(ratelimit, 32 >> (PAGE_SHIFT - 10));
 
-        current->nr_dirtied += nr_pages_dirtied;
-
         preempt_disable();
         /*
          * This prevents one CPU to accumulate too many dirtied pages without
@@ -1270,12 +1268,9 @@ void balance_dirty_pages_ratelimited_nr(struct address_space *mapping,
         p =  &__get_cpu_var(bdp_ratelimits);
         if (unlikely(current->nr_dirtied >= ratelimit))
                 *p = 0;
-        else {
-                *p += nr_pages_dirtied;
-                if (unlikely(*p >= ratelimit_pages)) {
-                        *p = 0;
-                        ratelimit = 0;
-                }
+        else if (unlikely(*p >= ratelimit_pages)) {
+                *p = 0;
+                ratelimit = 0;
         }
         /*
          * Pick up the dirtied pages by the exited tasks. This avoids lots of
@@ -1768,6 +1763,8 @@ void account_page_dirtied(struct page *page, struct address_space *mapping)
                 __inc_bdi_stat(mapping->backing_dev_info, BDI_RECLAIMABLE);
                 __inc_bdi_stat(mapping->backing_dev_info, BDI_DIRTIED);
                 task_io_account_write(PAGE_CACHE_SIZE);
+                current->nr_dirtied++;
+                this_cpu_inc(bdp_ratelimits);
         }
 }
 EXPORT_SYMBOL(account_page_dirtied);