Btrfs: honor extent thresh during defragmentation

We won't defrag an extent, if it's bigger than the threshold we
specified and there's no small extent before it, but actually
the code doesn't work this way.

There are three bugs:

- When should_defrag_range() decides we should keep on defragmenting
  an extent, last_len is not incremented. (old bug)

- The length that passes to should_defrag_range() is not the length
  we're going to defrag. (new bug)

- We always defrag 256K bytes data, and a big extent can be part of
  this range. (new bug)

For a file with 4 extents:

        | 4K | 4K | 256K | 256K |

The result of defrag with (the default) 256K extent thresh should be:

        | 264K | 256K |

but with those bugs, we'll get:

        | 520K |
Signed-off-by: NLi Zefan <lizf@cn.fujitsu.com>

fs/btrfs/ioctl.c

@@ -765,7 +765,7 @@ static int should_defrag_range(struct inode *inode, u64 start, u64 len,
 	int ret = 1;
 
 	/*
-	 * make sure that once we start defragging and extent, we keep on
+	 * make sure that once we start defragging an extent, we keep on
 	 * defragging it
 	 */
 	if (start < *defrag_end)
@@ -810,7 +810,6 @@ static int should_defrag_range(struct inode *inode, u64 start, u64 len,
 	 * extent will force at least part of that big extent to be defragged.
 	 */
 	if (ret) {
-		*last_len += len;
 		*defrag_end = extent_map_end(em);
 	} else {
 		*last_len = 0;
@@ -984,13 +983,14 @@ int btrfs_defrag_file(struct inode *inode, struct file *file,
 	u64 skip = 0;
 	u64 defrag_end = 0;
 	u64 newer_off = range->start;
-	int newer_left = 0;
 	unsigned long i;
+	unsigned long ra_index = 0;
 	int ret;
 	int defrag_count = 0;
 	int compress_type = BTRFS_COMPRESS_ZLIB;
 	int extent_thresh = range->extent_thresh;
-	int newer_cluster = (256 * 1024) >> PAGE_CACHE_SHIFT;
+	int max_cluster = (256 * 1024) >> PAGE_CACHE_SHIFT;
+	int cluster = max_cluster;
 	u64 new_align = ~((u64)128 * 1024 - 1);
 	struct page **pages = NULL;
 
@@ -1020,7 +1020,7 @@ int btrfs_defrag_file(struct inode *inode, struct file *file,
 		ra = &file->f_ra;
 	}
 
-	pages = kmalloc(sizeof(struct page *) * newer_cluster,
+	pages = kmalloc(sizeof(struct page *) * max_cluster,
 			GFP_NOFS);
 	if (!pages) {
 		ret = -ENOMEM;
@@ -1045,7 +1045,6 @@ int btrfs_defrag_file(struct inode *inode, struct file *file,
 			 * the extents in the file evenly spaced
 			 */
 			i = (newer_off & new_align) >> PAGE_CACHE_SHIFT;
-			newer_left = newer_cluster;
 		} else
 			goto out_ra;
 	} else {
@@ -1077,12 +1076,26 @@ int btrfs_defrag_file(struct inode *inode, struct file *file,
 			i = max(i + 1, next);
 			continue;
 		}
+
+		if (!newer_than) {
+			cluster = (PAGE_CACHE_ALIGN(defrag_end) >>
+				   PAGE_CACHE_SHIFT) - i;
+			cluster = min(cluster, max_cluster);
+		} else {
+			cluster = max_cluster;
+		}
+
 		if (range->flags & BTRFS_DEFRAG_RANGE_COMPRESS)
 			BTRFS_I(inode)->force_compress = compress_type;
 
-		btrfs_force_ra(inode->i_mapping, ra, file, i, newer_cluster);
+		if (i + cluster > ra_index) {
+			ra_index = max(i, ra_index);
+			btrfs_force_ra(inode->i_mapping, ra, file, ra_index,
+				       cluster);
+			ra_index += max_cluster;
+		}
 
-		ret = cluster_pages_for_defrag(inode, pages, i, newer_cluster);
+		ret = cluster_pages_for_defrag(inode, pages, i, cluster);
 		if (ret < 0)
 			goto out_ra;
 
@@ -1102,15 +1115,17 @@ int btrfs_defrag_file(struct inode *inode, struct file *file,
 			if (!ret) {
 				range->start = newer_off;
 				i = (newer_off & new_align) >> PAGE_CACHE_SHIFT;
-				newer_left = newer_cluster;
 			} else {
 				break;
 			}
 		} else {
-			if (ret > 0)
+			if (ret > 0) {
 				i += ret;
-			else
+				last_len += ret << PAGE_CACHE_SHIFT;
+			} else {
 				i++;
+				last_len = 0;
+			}
 		}
 	}