ext4: do not normalize block requests from fallocate()

Currently, an fallocate request of size slightly larger than a power of
2 is turned into two block requests, each a power of 2, with the extra
blocks pre-allocated for future use. When an application calls
fallocate, it already has an idea about how large the file may grow so
there is usually little benefit to reserve extra blocks on the
preallocation list. This reduces disk fragmentation.

Tested: fsstress. Also verified manually that fallocat'ed files are
contiguously laid out with this change (whereas without it they begin at
power-of-2 boundaries, leaving blocks in between). CPU usage of
fallocate is not appreciably higher.  In a tight fallocate loop, CPU
usage hovers between 5%-8% with this change, and 5%-7% without it.

Using a simulated file system aging program which the file system to
70%, the percentage of free extents larger than 8MB (as measured by
e2freefrag) increased from 38.8% without this change, to 69.4% with
this change.

Signed-off-by: Vivek Haldar <haldar@google.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/ext4.h

@@ -517,6 +517,8 @@ struct ext4_new_group_data {
 					 EXT4_GET_BLOCKS_CREATE_UNINIT_EXT)
 	/* Punch out blocks of an extent */
 #define EXT4_GET_BLOCKS_PUNCH_OUT_EXT		0x0020
+	/* Don't normalize allocation size (used for fallocate) */
+#define EXT4_GET_BLOCKS_NO_NORMALIZE		0x0040
 
 /*
  * Flags used by ext4_free_blocks

fs/ext4/extents.c

@@ -3554,6 +3554,8 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
 	else
 		/* disable in-core preallocation for non-regular files */
 		ar.flags = 0;
+	if (flags & EXT4_GET_BLOCKS_NO_NORMALIZE)
+		ar.flags |= EXT4_MB_HINT_NOPREALLOC;
 	newblock = ext4_mb_new_blocks(handle, &ar, &err);
 	if (!newblock)
 		goto out2;
@@ -3807,7 +3809,8 @@ long ext4_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
 			break;
 		}
 		ret = ext4_map_blocks(handle, inode, &map,
-				      EXT4_GET_BLOCKS_CREATE_UNINIT_EXT);
+				      EXT4_GET_BLOCKS_CREATE_UNINIT_EXT |
+				      EXT4_GET_BLOCKS_NO_NORMALIZE);
 		if (ret <= 0) {
 #ifdef EXT4FS_DEBUG
 			WARN_ON(ret <= 0);