bcachefs: bch_member->seq

Add new fields for split brain detection:

 - bch_member->seq, which tracks the sequence number of the last superblock
   write that happened to each member device

 - bch_sb->write_time, which tracks the time of the last superblock write,
   to allow detection of when two members have diverged but had the same
   number of superblock writes.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bcachefs_format.h

@@ -1303,6 +1303,7 @@ struct bch_member {
 	__le64			errors[BCH_MEMBER_ERROR_NR];
 	__le64			errors_at_reset[BCH_MEMBER_ERROR_NR];
 	__le64			errors_reset_time;
+	__le64			seq;
 };
 
 #define BCH_MEMBER_V1_BYTES	56
@@ -1729,7 +1730,9 @@ struct bch_sb_field_downgrade {
 	x(deleted_inodes,		BCH_VERSION(1,  2),		\
 	  BIT_ULL(BCH_RECOVERY_PASS_check_inodes))			\
 	x(rebalance_work,		BCH_VERSION(1,  3),		\
-	  BIT_ULL(BCH_RECOVERY_PASS_set_fs_needs_rebalance))
+	  BIT_ULL(BCH_RECOVERY_PASS_set_fs_needs_rebalance))		\
+	x(member_seq,			BCH_VERSION(1,  4),		\
+	  0)
 
 enum bcachefs_metadata_version {
 	bcachefs_metadata_version_min = 9,
@@ -1795,7 +1798,8 @@ struct bch_sb {
 	__le32			time_base_hi;
 	__le32			time_precision;
 
-	__le64			flags[8];
+	__le64			flags[7];
+	__le64			write_time;
 	__le64			features[2];
 	__le64			compat[2];
 

fs/bcachefs/sb-members.c

@@ -235,6 +235,11 @@ static void member_to_text(struct printbuf *out,
 		prt_printf(out, "(never)");
 	prt_newline(out);
 
+	prt_printf(out, "Last superblock write:");
+	prt_tab(out);
+	prt_u64(out, le64_to_cpu(m.seq));
+	prt_newline(out);
+
 	prt_printf(out, "State:");
 	prt_tab(out);
 	prt_printf(out, "%s",

fs/bcachefs/super-io.c

@@ -564,6 +564,7 @@ static int __copy_super(struct bch_sb_handle *dst_handle, struct bch_sb *src)
 	dst->time_base_lo	= src->time_base_lo;
 	dst->time_base_hi	= src->time_base_hi;
 	dst->time_precision	= src->time_precision;
+	dst->write_time		= src->write_time;
 
 	memcpy(dst->flags,	src->flags,	sizeof(dst->flags));
 	memcpy(dst->features,	src->features,	sizeof(dst->features));
@@ -942,6 +943,11 @@ int bch2_write_super(struct bch_fs *c)
 
 	le64_add_cpu(&c->disk_sb.sb->seq, 1);
 
+	struct bch_sb_field_members_v2 *mi = bch2_sb_field_get(c->disk_sb.sb, members_v2);
+	for_each_online_member(c, ca)
+		__bch2_members_v2_get_mut(mi, ca->dev_idx)->seq = c->disk_sb.sb->seq;
+	c->disk_sb.sb->write_time = cpu_to_le64(ktime_get_real_seconds());
+
 	if (test_bit(BCH_FS_error, &c->flags))
 		SET_BCH_SB_HAS_ERRORS(c->disk_sb.sb, 1);
 	if (test_bit(BCH_FS_topology_error, &c->flags))
@@ -1304,6 +1310,11 @@ void bch2_sb_to_text(struct printbuf *out, struct bch_sb *sb,
 	prt_printf(out, "%llu", le64_to_cpu(sb->seq));
 	prt_newline(out);
 
+	prt_printf(out, "Time of last write:");
+	prt_tab(out);
+	bch2_prt_datetime(out, le64_to_cpu(sb->write_time));
+	prt_newline(out);
+
 	prt_printf(out, "Superblock size:");
 	prt_tab(out);
 	prt_printf(out, "%zu", vstruct_bytes(sb));