bcachefs: BCH_SB_VERSION_INCOMPAT

We've been getting away from feature bits: they don't have any kind of
ordering, and thus it's possible for people to enable weird combinations
of features that were never tested or intended to be run.

Much better to just give every new feature, compatible or incompatible,
a version number.

Additionally, we probably won't ever rev the major version number: major
version numbers represent incompatible versions, but that doesn't really
fit with how we actually roll out incompatible features - we need a
better way of rolling out incompatible features.

So, this patch adds two new superblock fields:
- BCH_SB_VERSION_INCOMPAT
- BCH_SB_VERSION_INCOMPAT_ALLOWED

BCH_SB_VERSION_INCOMPAT_ALLOWED indicates that incompatible features up
to version number x are allowed to be used without user prompting, but
it does not by itself deny old versions from mounting.

BCH_SB_VERSION_INCOMPAT does deny old versions from mounting, and must
be <= BCH_SB_VERSION_INCOMPAT_ALLOWED.

BCH_SB_VERSION_INCOMPAT will only be set when a codepath attempts to use
an incompatible feature, so as to not unnecessarily break compatibility
with old versions.

bch2_request_incompat_feature() is the new interface to check if an
incompatible feature may be used.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

1 files changed, 16 insertions, 3 deletions

diff --git a/fs/bcachefs/super-io.h b/fs/bcachefs/super-io.h
index 90e7b176cdd0..f1ab4f943720 100644
--- a/fs/bcachefs/super-io.h
+++ b/fs/bcachefs/super-io.h
@@ -18,8 +18,21 @@ static inline bool bch2_version_compatible(u16 version)
 		version >= bcachefs_metadata_version_min;
 }
 
-void bch2_version_to_text(struct printbuf *, unsigned);
-unsigned bch2_latest_compatible_version(unsigned);
+void bch2_version_to_text(struct printbuf *, enum bcachefs_metadata_version);
+enum bcachefs_metadata_version bch2_latest_compatible_version(enum bcachefs_metadata_version);
+
+void bch2_set_version_incompat(struct bch_fs *, enum bcachefs_metadata_version);
+
+static inline bool bch2_request_incompat_feature(struct bch_fs *c,
+						 enum bcachefs_metadata_version version)
+{
+	if (unlikely(version > c->sb.version_incompat)) {
+		if (version > c->sb.version_incompat_allowed)
+			return false;
+		bch2_set_version_incompat(c, version);
+	}
+	return true;
+}
 
 static inline size_t bch2_sb_field_bytes(struct bch_sb_field *f)
 {
@@ -94,7 +107,7 @@ static inline void bch2_check_set_feature(struct bch_fs *c, unsigned feat)
 }
 
 bool bch2_check_version_downgrade(struct bch_fs *);
-void bch2_sb_upgrade(struct bch_fs *, unsigned);
+void bch2_sb_upgrade(struct bch_fs *, unsigned, bool);
 
 void __bch2_sb_field_to_text(struct printbuf *, struct bch_sb *,
 			     struct bch_sb_field *);