[POC for v5.15 2/2] btrfs: defrag: limit cluster size to the first hole/prealloc range

[Qu Wenruo <wqu@suse.com>]

We always use 256K as cluster size if possible, but it's very common
that there are only several small regular extents which are target for
defrag.

But those extents are not filling the whole 256K cluster, and the holes
after those extents will make defrag to reject the whole cluster.

This patch will mitigate the problem by doing another search for the
hole/preallocated range, and limit the size of the cluster just to the
regular extents part.

By this, defrag can do more of its work without being interrupted by a
hole.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/ioctl.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 8350864a4bd8..b66bb10e2e4a 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -1481,6 +1481,29 @@ static int cluster_pages_for_defrag(struct inode *inode,
 
 }
 
+static u64 get_cluster_size(struct inode *inode, u64 start, u64 len)
+{
+	u64 cur = start;
+	u64 cluster_len = 0;
+	while (cur < start + len) {
+		struct extent_map *em;
+
+		em = defrag_lookup_extent(inode, cur, false);
+		if (!em)
+			break;
+		/*
+		 * Here we don't do comprehensive checks, we just
+		 * find the first hole/preallocated.
+		 */
+		if (em->block_start >= EXTENT_MAP_LAST_BYTE ||
+		    test_bit(EXTENT_FLAG_PREALLOC, &em->flags))
+			break;
+		cluster_len += min(em->start + em->len - cur, start + len - cur);
+		cur = min(em->start + em->len, start + len);
+	}
+	return cluster_len;
+}
+
 /*
  * Entry point to file defragmentation.
  *
@@ -1618,6 +1641,13 @@ int btrfs_defrag_file(struct inode *inode, struct file_ra_state *ra,
 		} else {
 			cluster = max_cluster;
 		}
+		cluster = min_t(unsigned long,
+				get_cluster_size(inode, i << PAGE_SHIFT,
+						 cluster << PAGE_SHIFT)
+					>> PAGE_SHIFT,
+			        cluster);
+		if (cluster == 0)
+			goto next;
 
 		if (i + cluster > ra_index) {
 			ra_index = max(i, ra_index);
@@ -1644,6 +1674,7 @@ int btrfs_defrag_file(struct inode *inode, struct file_ra_state *ra,
 		balance_dirty_pages_ratelimited(inode->i_mapping);
 		btrfs_inode_unlock(inode, 0);
 
+next:
 		if (newer_than) {
 			if (newer_off == (u64)-1)
 				break;
-- 
2.34.1