bcachefs: bch_sb_field_downgrade

Add a new superblock section that contains a list of
  { minor version, recovery passes, errors_to_fix }

that is - a list of recovery passes that must be run when downgrading
past a given version, and a list of errors to silently fix.

The upcoming disk accounting rewrite is not going to be fully
compatible: we're going to have to regenerate accounting both when
upgrading to the new version, and also from downgrading from the new
version, since the new method of doing disk space accounting is a
completely different architecture based on deltas, and synchronizing
them for every jounal entry write to maintain compatibility is going to
be too expensive and impractical.

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

1 files changed, 10 insertions, 1 deletions

diff --git a/fs/bcachefs/super-io.c b/fs/bcachefs/super-io.c
index e085d3b021e8..4c98d8cc2a79 100644
--- a/fs/bcachefs/super-io.c
+++ b/fs/bcachefs/super-io.c
@@ -13,6 +13,7 @@
 #include "replicas.h"
 #include "quota.h"
 #include "sb-clean.h"
+#include "sb-downgrade.h"
 #include "sb-errors.h"
 #include "sb-members.h"
 #include "super-io.h"
@@ -939,6 +940,7 @@ int bch2_write_super(struct bch_fs *c)
 	bch2_sb_members_from_cpu(c);
 	bch2_sb_members_cpy_v2_v1(&c->disk_sb);
 	bch2_sb_errors_from_cpu(c);
+	bch2_sb_downgrade_update(c);
 
 	for_each_online_member(ca, c, i)
 		bch2_sb_from_fs(c, ca);
@@ -1062,8 +1064,10 @@ void __bch2_check_set_feature(struct bch_fs *c, unsigned feat)
 }
 
 /* Downgrade if superblock is at a higher version than currently supported: */
-void bch2_sb_maybe_downgrade(struct bch_fs *c)
+bool bch2_check_version_downgrade(struct bch_fs *c)
 {
+	bool ret = bcachefs_metadata_version_current < c->sb.version;
+
 	lockdep_assert_held(&c->sb_lock);
 
 	/*
@@ -1077,12 +1081,17 @@ void bch2_sb_maybe_downgrade(struct bch_fs *c)
 	if (c->sb.version_min > bcachefs_metadata_version_current)
 		c->disk_sb.sb->version_min = cpu_to_le16(bcachefs_metadata_version_current);
 	c->disk_sb.sb->compat[0] &= cpu_to_le64((1ULL << BCH_COMPAT_NR) - 1);
+	return ret;
 }
 
 void bch2_sb_upgrade(struct bch_fs *c, unsigned new_version)
 {
 	lockdep_assert_held(&c->sb_lock);
 
+	if (BCH_VERSION_MAJOR(new_version) >
+	    BCH_VERSION_MAJOR(le16_to_cpu(c->disk_sb.sb->version)))
+		bch2_sb_field_resize(&c->disk_sb, downgrade, 0);
+
 	c->disk_sb.sb->version = cpu_to_le16(new_version);
 	c->disk_sb.sb->features[0] |= cpu_to_le64(BCH_SB_FEATURES_ALL);
 }