Partially bring r242520 to ext2fs.

When a file is first being written, the dynamic block reallocation
(implemented by ext2_reallocblks) relocates the file's blocks
so as to cluster them together into a contiguous set of blocks on
the disk.

When the cluster crosses the boundary into the first indirect block,
the first indirect block is initially allocated in a position
immediately following the last direct block.  Block reallocation
would usually destroy locality by moving the indirect block out of
the way to keep the data blocks contiguous.

The issue was diagnosed long ago by Bruce Evans on ffs and surfaced
on ext2fs when block reallocaton was ported. This is only a partial
solution based on the similarities with FFS. We still require more
review of the allocation details that vary in ext2fs.

Reported by:	bde
MFC after:	1 week

1 files changed, 12 insertions, 0 deletions

diff --git a/sys/fs/ext2fs/ext2_alloc.c b/sys/fs/ext2fs/ext2_alloc.c
index 7867a87..a701087 100644
--- a/sys/fs/ext2fs/ext2_alloc.c
+++ b/sys/fs/ext2fs/ext2_alloc.c
@@ -197,6 +197,18 @@ ext2_reallocblks(ap)
 			panic("ext2_reallocblks: non-cluster");
 #endif
 	/*
+	 * If the cluster crosses the boundary for the first indirect
+	 * block, leave space for the indirect block. Indirect blocks
+	 * are initially laid out in a position after the last direct
+	 * block. Block reallocation would usually destroy locality by
+	 * moving the indirect block out of the way to make room for
+	 * data blocks if we didn't compensate here. We should also do
+	 * this for other indirect block boundaries, but it is only
+	 * important for the first one.
+	 */
+	if (start_lbn < NDADDR && end_lbn >= NDADDR)
+		return (ENOSPC);
+	/*
 	 * If the latest allocation is in a new cylinder group, assume that
 	 * the filesystem has decided to move and do not force it back to
 	 * the previous cylinder group.