md: bitmap: improve bitmap maintenance code.

The code for checking which bits in the bitmap can be cleared
has 2 problems:
 1/ it repeatedly takes and drops a spinlock, where it would make
    more sense to just hold on to it most of the time.
 2/ it doesn't make use of some opportunities to skip large sections
    of the bitmap

This patch fixes those.  It will only affect CPU consumption, not
correctness.

Signed-off-by: NeilBrown <neilb@suse.de>

1 files changed, 7 insertions, 6 deletions

diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c
index 47c68bc75a17..56df1cee8fb3 100644
--- a/drivers/md/bitmap.c
+++ b/drivers/md/bitmap.c
@@ -1097,14 +1097,12 @@ void bitmap_daemon_work(struct bitmap *bitmap)
         }
         bitmap->allclean = 1;
 
+        spin_lock_irqsave(&bitmap->lock, flags);
         for (j = 0; j < bitmap->chunks; j++) {
                 bitmap_counter_t *bmc;
-                spin_lock_irqsave(&bitmap->lock, flags);
-                if (!bitmap->filemap) {
+                if (!bitmap->filemap)
                         /* error or shutdown */
-                        spin_unlock_irqrestore(&bitmap->lock, flags);
                         break;
-                }
 
                 page = filemap_get_page(bitmap, j);
 
@@ -1121,6 +1119,8 @@ void bitmap_daemon_work(struct bitmap *bitmap)
                                         write_page(bitmap, page, 0);
                                         bitmap->allclean = 0;
                                 }
+                                spin_lock_irqsave(&bitmap->lock, flags);
+                                j |= (PAGE_BITS - 1);
                                 continue;
                         }
 
@@ -1181,9 +1181,10 @@ void bitmap_daemon_work(struct bitmap *bitmap)
                                         ext2_clear_bit(file_page_offset(j), paddr);
                                 kunmap_atomic(paddr, KM_USER0);
                         }
-                }
-                spin_unlock_irqrestore(&bitmap->lock, flags);
+                } else
+                        j |= PAGE_COUNTER_MASK;
         }
+        spin_unlock_irqrestore(&bitmap->lock, flags);
 
         /* now sync the final page */
         if (lastpage != NULL) {