ext4: make the preallocation size be a multiple of stripe size

Previously, if a stripe width was provided, then it would be used
as the preallocation granularity, with no santiy checking and no
way to override this. Now, mb_prealloc_size defaults to the smallest
multiple of stripe size that is greater than or equal to the old
default mb_prealloc_size, and this can be overridden with the sysfs
interface.
Signed-off-by: NDan Ehrenberg <dehrenberg@google.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/mballoc.c

@@ -128,12 +128,13 @@
  * we are doing a group prealloc we try to normalize the request to
  * sbi->s_mb_group_prealloc. Default value of s_mb_group_prealloc is
  * 512 blocks. This can be tuned via
- * /sys/fs/ext4/<partition/mb_group_prealloc. The value is represented in
+ * /sys/fs/ext4/<partition>/mb_group_prealloc. The value is represented in
  * terms of number of blocks. If we have mounted the file system with -O
  * stripe=<value> option the group prealloc request is normalized to the
- * stripe value (sbi->s_stripe)
+ * the smallest multiple of the stripe value (sbi->s_stripe) which is
+ * greater than the default mb_group_prealloc.
  *
- * The regular allocator(using the buddy cache) supports few tunables.
+ * The regular allocator (using the buddy cache) supports a few tunables.
  *
  * /sys/fs/ext4/<partition>/mb_min_to_scan
  * /sys/fs/ext4/<partition>/mb_max_to_scan
@@ -2474,6 +2475,18 @@ int ext4_mb_init(struct super_block *sb, int needs_recovery)
 	sbi->s_mb_stream_request = MB_DEFAULT_STREAM_THRESHOLD;
 	sbi->s_mb_order2_reqs = MB_DEFAULT_ORDER2_REQS;
 	sbi->s_mb_group_prealloc = MB_DEFAULT_GROUP_PREALLOC;
+	/*
+	 * If there is a s_stripe > 1, then we set the s_mb_group_prealloc
+	 * to the lowest multiple of s_stripe which is bigger than
+	 * the s_mb_group_prealloc as determined above. We want
+	 * the preallocation size to be an exact multiple of the
+	 * RAID stripe size so that preallocations don't fragment
+	 * the stripes.
+	 */
+	if (sbi->s_stripe > 1) {
+		sbi->s_mb_group_prealloc = roundup(
+			sbi->s_mb_group_prealloc, sbi->s_stripe);
+	}
 
 	sbi->s_locality_groups = alloc_percpu(struct ext4_locality_group);
 	if (sbi->s_locality_groups == NULL) {
@@ -2841,8 +2854,9 @@ ext4_mb_mark_diskspace_used(struct ext4_allocation_context *ac,
 
 /*
  * here we normalize request for locality group
- * Group request are normalized to s_strip size if we set the same via mount
- * option. If not we set it to s_mb_group_prealloc which can be configured via
+ * Group request are normalized to s_mb_group_prealloc, which goes to
+ * s_strip if we set the same via mount option.
+ * s_mb_group_prealloc can be configured via
  * /sys/fs/ext4/<partition>/mb_group_prealloc
  *
  * XXX: should we try to preallocate more than the group has now?
@@ -2853,10 +2867,7 @@ static void ext4_mb_normalize_group_request(struct ext4_allocation_context *ac)
 	struct ext4_locality_group *lg = ac->ac_lg;
 
 	BUG_ON(lg == NULL);
-	if (EXT4_SB(sb)->s_stripe)
-		ac->ac_g_ex.fe_len = EXT4_SB(sb)->s_stripe;
-	else
-		ac->ac_g_ex.fe_len = EXT4_SB(sb)->s_mb_group_prealloc;
+	ac->ac_g_ex.fe_len = EXT4_SB(sb)->s_mb_group_prealloc;
 	mb_debug(1, "#%u: goal %u blocks for locality group\n",
 		current->pid, ac->ac_g_ex.fe_len);
 }