md/bitmap: improve handling of 'allclean'.

The 'allclean' flag is used to cache the fact that there is nothing to
do, so we can avoid waking up and scanning the bitmap regularly.

The two sorts of pages that might need the attention of the bitmap
daemon are BITMAP_PAGE_PENDING and BITMAP_PAGE_NEEDWRITE pages.

So make sure allclean reflects exactly when there are none of those.
So:
  set it before scanning all pages with either bit set.
  clear it whenever these bits are set
  clear it when we desire not to clear one of these bits.
  don't clear it any other time.
Signed-off-by: NNeilBrown <neilb@suse.de>

drivers/md/bitmap.c

@@ -1146,6 +1146,7 @@ void bitmap_write_all(struct bitmap *bitmap)
 	for (i = 0; i < bitmap->file_pages; i++)
 		set_page_attr(bitmap, bitmap->filemap[i],
 			      BITMAP_PAGE_NEEDWRITE);
+	bitmap->allclean = 0;
 }
 
 static void bitmap_count_page(struct bitmap *bitmap, sector_t offset, int inc)
@@ -1211,10 +1212,8 @@ void bitmap_daemon_work(mddev_t *mddev)
 					clear_page_attr(bitmap, page, BITMAP_PAGE_NEEDWRITE);
 
 				spin_unlock_irqrestore(&bitmap->lock, flags);
-				if (need_write) {
+				if (need_write)
 					write_page(bitmap, page, 0);
-					bitmap->allclean = 0;
-				}
 				spin_lock_irqsave(&bitmap->lock, flags);
 				j |= (PAGE_BITS - 1);
 				continue;
@@ -1222,12 +1221,16 @@ void bitmap_daemon_work(mddev_t *mddev)
 
 			/* grab the new page, sync and release the old */
 			if (lastpage != NULL) {
-				if (test_page_attr(bitmap, lastpage, BITMAP_PAGE_NEEDWRITE)) {
-					clear_page_attr(bitmap, lastpage, BITMAP_PAGE_NEEDWRITE);
+				if (test_page_attr(bitmap, lastpage,
+						   BITMAP_PAGE_NEEDWRITE)) {
+					clear_page_attr(bitmap, lastpage,
+							BITMAP_PAGE_NEEDWRITE);
 					spin_unlock_irqrestore(&bitmap->lock, flags);
 					write_page(bitmap, lastpage, 0);
 				} else {
-					set_page_attr(bitmap, lastpage, BITMAP_PAGE_NEEDWRITE);
+					set_page_attr(bitmap, lastpage,
+						      BITMAP_PAGE_NEEDWRITE);
+					bitmap->allclean = 0;
 					spin_unlock_irqrestore(&bitmap->lock, flags);
 				}
 			} else
@@ -1250,6 +1253,8 @@ void bitmap_daemon_work(mddev_t *mddev)
 			spin_lock_irqsave(&bitmap->lock, flags);
 			if (!bitmap->need_sync)
 				clear_page_attr(bitmap, page, BITMAP_PAGE_PENDING);
+			else
+				bitmap->allclean = 0;
 		}
 		bmc = bitmap_get_counter(bitmap,
 					 (sector_t)j << CHUNK_BLOCK_SHIFT(bitmap),
@@ -1257,8 +1262,6 @@ void bitmap_daemon_work(mddev_t *mddev)
 		if (!bmc)
 			j |= PAGE_COUNTER_MASK;
 		else if (*bmc) {
-			bitmap->allclean = 0;
-
 			if (*bmc == 1 && !bitmap->need_sync) {
 				/* we can clear the bit */
 				*bmc = 0;
@@ -1280,6 +1283,7 @@ void bitmap_daemon_work(mddev_t *mddev)
 			} else if (*bmc <= 2) {
 				*bmc = 1; /* maybe clear the bit next time */
 				set_page_attr(bitmap, page, BITMAP_PAGE_PENDING);
+				bitmap->allclean = 0;
 			}
 		}
 	}
@@ -1294,6 +1298,7 @@ void bitmap_daemon_work(mddev_t *mddev)
 			write_page(bitmap, lastpage, 0);
 		} else {
 			set_page_attr(bitmap, lastpage, BITMAP_PAGE_NEEDWRITE);
+			bitmap->allclean = 0;
 			spin_unlock_irqrestore(&bitmap->lock, flags);
 		}
 	}
@@ -1407,7 +1412,6 @@ int bitmap_startwrite(struct bitmap *bitmap, sector_t offset, unsigned long sect
 		else
 			sectors = 0;
 	}
-	bitmap->allclean = 0;
 	return 0;
 }
 EXPORT_SYMBOL(bitmap_startwrite);
@@ -1453,13 +1457,14 @@ void bitmap_endwrite(struct bitmap *bitmap, sector_t offset, unsigned long secto
 			wake_up(&bitmap->overflow_wait);
 
 		(*bmc)--;
-		if (*bmc <= 2)
+		if (*bmc <= 2) {
 			set_page_attr(bitmap,
 				      filemap_get_page(
 					      bitmap,
 					      offset >> CHUNK_BLOCK_SHIFT(bitmap)),
 				      BITMAP_PAGE_PENDING);
-
+			bitmap->allclean = 0;
+		}
 		spin_unlock_irqrestore(&bitmap->lock, flags);
 		offset += blocks;
 		if (sectors > blocks)
@@ -1495,7 +1500,6 @@ static int __bitmap_start_sync(struct bitmap *bitmap, sector_t offset, sector_t
 		}
 	}
 	spin_unlock_irq(&bitmap->lock);
-	bitmap->allclean = 0;
 	return rv;
 }
 
@@ -1543,15 +1547,16 @@ void bitmap_end_sync(struct bitmap *bitmap, sector_t offset, sector_t *blocks, i
 		if (!NEEDED(*bmc) && aborted)
 			*bmc |= NEEDED_MASK;
 		else {
-			if (*bmc <= 2)
+			if (*bmc <= 2) {
 				set_page_attr(bitmap,
 					      filemap_get_page(bitmap, offset >> CHUNK_BLOCK_SHIFT(bitmap)),
 					      BITMAP_PAGE_PENDING);
+				bitmap->allclean = 0;
+			}
 		}
 	}
  unlock:
 	spin_unlock_irqrestore(&bitmap->lock, flags);
-	bitmap->allclean = 0;
 }
 EXPORT_SYMBOL(bitmap_end_sync);
 
@@ -1623,9 +1628,9 @@ static void bitmap_set_memory_bits(struct bitmap *bitmap, sector_t offset, int n
 		bitmap_count_page(bitmap, offset, 1);
 		page = filemap_get_page(bitmap, offset >> CHUNK_BLOCK_SHIFT(bitmap));
 		set_page_attr(bitmap, page, BITMAP_PAGE_PENDING);
+		bitmap->allclean = 0;
 	}
 	spin_unlock_irq(&bitmap->lock);
-	bitmap->allclean = 0;
 }
 
 /* dirty the memory and file bits for bitmap chunks "s" to "e" */