btrfs: fix dirtied pages accounting on sub-page writes

When doing 1KB sequential writes to the same page,
balance_dirty_pages_ratelimited_nr() should be called once instead of 4
times, the latter makes the dirtier tasks be throttled much too heavy.

Fix it with proper de-accounting on clear_page_dirty_for_io().

CC: Chris Mason <chris.mason@oracle.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 97fbe93..bfb620e 100644 (file)
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1136,7 +1136,8 @@ again:
                                     GFP_NOFS);
        }
        for (i = 0; i < num_pages; i++) {
-               clear_page_dirty_for_io(pages[i]);
+               if (clear_page_dirty_for_io(pages[i]))
+                       account_page_redirty(pages[i]);
                set_page_extent_mapped(pages[i]);
                WARN_ON(!PageLocked(pages[i]));
        }