mkfs.btrfs: enabling only running kernel compatible features

[asj]

Binaries compiled from btrfs-progs source code have been compatible with older kernels if users use the -O option with mkfs.btrfs and specify the compatible features manually. Otherwise, a latest mkfs.btrfs may enable some features by default, even if the current kernel doesn't support them.

I propose a sub-option to enable only compatible features: mkfs.btrfs --features=by-sysfs <dev> by reading the sysfs.

List of on-disk format changes incompatible with previous kernel version unless feature was back-ported; attempting to mount on older kernel will fail.

RW incompatible:

mixed_backref   BTRFS_FEATURE_INCOMPAT_MIXED_BACKREF
default_subvol  BTRFS_FEATURE_INCOMPAT_DEFAULT_SUBVOL
mixed_groups    BTRFS_FEATURE_INCOMPAT_MIXED_GROUPS
compress_lzo    BTRFS_FEATURE_INCOMPAT_COMPRESS_LZO
BTRFS_FEATURE_INCOMPAT_COMPRESS_LZOv2
big_metadata    BTRFS_FEATURE_INCOMPAT_BIG_METADATA
extended_iref   BTRFS_FEATURE_INCOMPAT_EXTENDED_IREF
raid56          BTRFS_FEATURE_INCOMPAT_RAID56
skinny_metadata BTRFS_FEATURE_INCOMPAT_SKINNY_METADATA
no_holes        BTRFS_FEATURE_INCOMPAT_NO_HOLES
zstd compression
metadata-uuid   BTRFS_FEATURE_INCOMPAT_METADATA_UUID
raid1c34        BTRFS_FEATURE_INCOMPAT_RAID1C34
zone-device     BTRFS_FEATURE_INCOMPAT_ZONED
read support of blocksize<pagesize (subpage)
write support of blocksize<pagesize (subpage)
extent-tree-v2  BTRFS_FEATURE_INCOMPAT_EXTENT_TREE_V2

RO compatible only:

BTRFS_FEATURE_COMPAT_RO_FREE_SPACE_TREE
BTRFS_FEATURE_COMPAT_RO_FREE_SPACE_TREE_VALID
fsverity support BTRFS_FEATURE_COMPAT_RO_VERITY
block group tree BTRFS_FEATURE_COMPAT_RO_BLOCK_GROUP_TREE

RW incompatible checksumming algorithms:

blake2b  checksumming
sha256   checksumming
xxhash64 checksumming

[hanyuwei70]

Agreed.
I have encoutered mkfs zoned RAID1 then can't mount them afterwards. (missing BTRFS_FEATURE_INCOMPAT_RAID_STRIPE_TREE )