Re: [PATCH] btrfs: loosen the block-group-tree feature dependency check

[David Sterba <dsterba@suse.cz>]

On Mon, Sep 12, 2022 at 01:44:37PM +0800, Qu Wenruo wrote:
> +int btrfs_check_features(struct btrfs_fs_info *fs_info, struct super_block *sb)
> +{
> +	struct btrfs_super_block *disk_super = fs_info->super_copy;
> +	u64 incompat = btrfs_super_incompat_flags(disk_super);
> +	u64 compat_ro = btrfs_super_compat_ro_flags(disk_super);
> +
> +	if (incompat & ~BTRFS_FEATURE_INCOMPAT_SUPP) {
> +		btrfs_err(fs_info,
> +		    "cannot mount because of unsupported optional features (0x%llx)",

I've reworded this as "cannot mount because of unknown incompat features
(0x%llx)"

> +		    incompat);
> +		return -EINVAL;
> +	}
> +
> +	/* Runtime limitation for mixed block groups. */
> +	if ((incompat & BTRFS_FEATURE_INCOMPAT_MIXED_GROUPS) &&
> +	    (fs_info->sectorsize != fs_info->nodesize)) {
> +		btrfs_err(fs_info,
> +"unequal nodesize/sectorsize (%u != %u) are not allowed for mixed block groups",
> +			fs_info->nodesize, fs_info->sectorsize);
> +		return -EINVAL;
> +	}
> +
> +	/* Mixed backref is an always-enabled feature. */
> +	incompat |= BTRFS_FEATURE_INCOMPAT_MIXED_BACKREF;
> +
> +	/* Set compression related flags just in case. */
> +	if (fs_info->compress_type == BTRFS_COMPRESS_LZO)
> +		incompat |= BTRFS_FEATURE_INCOMPAT_COMPRESS_LZO;
> +	else if (fs_info->compress_type == BTRFS_COMPRESS_ZSTD)
> +		incompat |= BTRFS_FEATURE_INCOMPAT_COMPRESS_ZSTD;
> +
> +	/*
> +	 * An ancient flag, which should really be marked deprecated.
> +	 * Such runtime limitation doesn't really need a incompat flag.
> +	 */
> +	if (btrfs_super_nodesize(disk_super) > PAGE_SIZE)
> +		incompat |= BTRFS_FEATURE_INCOMPAT_BIG_METADATA;
> +
> +	if (compat_ro & ~BTRFS_FEATURE_COMPAT_RO_SUPP && !sb_rdonly(sb)) {
> +		btrfs_err(fs_info,
> +	"cannot mount read-write because of unsupported optional features (0x%llx)",

and "cannot mount read-write because of unknown compat_ro features
(0x%llx)"

so it can be matched agains the incompat and compat_ro fields eg. in the
super block dump.