Re: [PATCH 5/6] btrfs: use raid_attr for minimum stripe count in btrfs_calc_avail_data_space

[Nikolay Borisov <nborisov@suse.com>]

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On 18.06.19 г. 21:00 ч., David Sterba wrote:
> Minimum stripe count matches the minimum devices required for a given
> profile. The open coded assignments match the raid_attr table.
> 
> What's changed here is the meaning for RAID5/6. Previously their
> min_stripes would be 1, while newly it's devs_min. This however shold be
> the same as before because it's not possible to create filesystem on
> fewer devices than the raid_attr table allows.
> 
> There's no adjustment regarding the parity stripes (like
> calc_data_stripes does), because we're interested in overall space that
> would fit on the devices.
> 
> Missing devices make no difference for the whole calculation, we have
> the size stored in the structures.

I think the clean up in this function should include more here's list of
things which I think will make it more readable. Something like the
attached diff on top of your patch:



[-- Attachment #2: calc-avail-space.diff --]
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diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 6e196b8a0820..230aef8314da 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1898,11 +1898,10 @@ static inline int btrfs_calc_avail_data_space(struct btrfs_fs_info *fs_info,
 	struct btrfs_device_info *devices_info;
 	struct btrfs_fs_devices *fs_devices = fs_info->fs_devices;
 	struct btrfs_device *device;
-	u64 skip_space;
 	u64 type;
 	u64 avail_space;
 	u64 min_stripe_size;
-	int min_stripes = 1, num_stripes = 1;
+	int num_stripes = 1;
 	int i = 0, nr_devices;
 
 	/*
@@ -1957,28 +1956,21 @@ static inline int btrfs_calc_avail_data_space(struct btrfs_fs_info *fs_info,
 		avail_space = device->total_bytes - device->bytes_used;
 
 		/* align with stripe_len */
-		avail_space = div_u64(avail_space, BTRFS_STRIPE_LEN);
-		avail_space *= BTRFS_STRIPE_LEN;
+		avail_space = rounddown(avail_space, BTRFS_STRIPE_LEN);
 
 		/*
 		 * In order to avoid overwriting the superblock on the drive,
 		 * btrfs starts at an offset of at least 1MB when doing chunk
 		 * allocation.
+		 *
+		 * This ensures we have at least min_stripe_size free space
+		 * after excluding 1mb.
 		 */
-		skip_space = SZ_1M;
-
-		/*
-		 * we can use the free space in [0, skip_space - 1], subtract
-		 * it from the total.
-		 */
-		if (avail_space && avail_space >= skip_space)
-			avail_space -= skip_space;
-		else
-			avail_space = 0;
-
-		if (avail_space < min_stripe_size)
+		if (avail_space <= SZ_1M + min_stripe_size)
 			continue;
 
+		avail_space -= SZ_1M;
+
 		devices_info[i].dev = device;
 		devices_info[i].max_avail = avail_space;
 
@@ -1992,9 +1984,8 @@ static inline int btrfs_calc_avail_data_space(struct btrfs_fs_info *fs_info,
 
 	i = nr_devices - 1;
 	avail_space = 0;
-	while (nr_devices >= min_stripes) {
-		if (num_stripes > nr_devices)
-			num_stripes = nr_devices;
+	while (nr_devices >= rattr->devs_min) {
+		num_stripes = min(num_stripes, nr_devices);
 
 		if (devices_info[i].max_avail >= min_stripe_size) {
 			int j;