[PATCH v3]btrfs: round down stripe size and chunk size to pow of 2

[Wang Yugui <wangyugui@e16-tech.com>]

In decide_stripe_size_regular(), when new disk is added to RAID0/RAID10/RAID56,
it is better to free-then-reuse the free space if stripe size is kept or
changed to 1/2. so stripe size of pow of 2 will be more friendly. Although
roundup_pow_of_two() match better with orig round_up(), but
rounddown_pow_of_two() is better to make sure <=ctl->max_chunk_size here.

In another rare case that file system is quite small, we calc max chunk size
in pow of 2 too, so that max chunk size / chunk size /stripe size are same or
match easy in some case.

Signed-off-by: Wang Yugui <wangyugui@e16-tech.com>
---
changes since v2:
	restore to rounddown_pow_of_two() from roundup_pow_of_two()
changes since v1:
 - change rounddown_pow_of_two() to roundup_pow_of_two() to match better with
   orig roundup().

 fs/btrfs/volumes.c | 20 +++++++++-----------
 1 file changed, 9 insertions(+), 11 deletions(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 6595755..fab9765 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -5083,9 +5083,9 @@ static void init_alloc_chunk_ctl_policy_regular(
 	if (ctl->type & BTRFS_BLOCK_GROUP_SYSTEM)
 		ctl->devs_max = min_t(int, ctl->devs_max, BTRFS_MAX_DEVS_SYS_CHUNK);
 
-	/* We don't want a chunk larger than 10% of writable space */
-	ctl->max_chunk_size = min(div_factor(fs_devices->total_rw_bytes, 1),
-				  ctl->max_chunk_size);
+	/* We don't want a chunk larger than 1/8 of writable space */
+	ctl->max_chunk_size = min_t(u64, ctl->max_chunk_size,
+			rounddown_pow_of_two(fs_devices->total_rw_bytes >> 3));
 	ctl->dev_extent_min = BTRFS_STRIPE_LEN * ctl->dev_stripes;
 }
 
@@ -5143,10 +5143,9 @@ static void init_alloc_chunk_ctl_policy_zoned(
 		BUG();
 	}
 
-	/* We don't want a chunk larger than 10% of writable space */
-	limit = max(round_down(div_factor(fs_devices->total_rw_bytes, 1),
-			       zone_size),
-		    min_chunk_size);
+	/* We don't want a chunk larger than 1/8 of writable space */
+	limit = max_t(u64, min_chunk_size,
+		rounddown_pow_of_two(fs_devices->total_rw_bytes >> 3));
 	ctl->max_chunk_size = min(limit, ctl->max_chunk_size);
 	ctl->dev_extent_min = zone_size * ctl->dev_stripes;
 }
@@ -5284,13 +5283,12 @@ static int decide_stripe_size_regular(struct alloc_chunk_ctl *ctl,
 	 */
 	if (ctl->stripe_size * data_stripes > ctl->max_chunk_size) {
 		/*
-		 * Reduce stripe_size, round it up to a 16MB boundary again and
+		 * Reduce stripe_size, round it down to pow of 2 boundary again and
 		 * then use it, unless it ends up being even bigger than the
 		 * previous value we had already.
 		 */
-		ctl->stripe_size = min(round_up(div_u64(ctl->max_chunk_size,
-							data_stripes), SZ_16M),
-				       ctl->stripe_size);
+		ctl->stripe_size = min_t(u64, ctl->stripe_size,
+			rounddown_pow_of_two(div_u64(ctl->max_chunk_size, data_stripes)));
 	}
 
 	/* Align to BTRFS_STRIPE_LEN */
-- 
2.36.2