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>

diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c
index 47c68bc..56df1ce 100644 (file)
--- 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) {