bcachefs: Fix a "no journal entries found" bug

On startup, we need to ensure the first journal entry written is a flush
write: after a clean shutdown we generally don't read the journal, which
means we might be overwriting whatever was there previously, and there
must always be at least one flush entry in the journal or recovery will
fail.

Found by fstests generic/388.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/journal_io.c

@@ -1661,20 +1661,42 @@ void bch2_journal_write(struct closure *cl)
 	j->write_start_time = local_clock();
 
 	spin_lock(&j->lock);
-	if (bch2_journal_error(j) ||
-	    w->noflush ||
-	    (!w->must_flush &&
-	     (jiffies - j->last_flush_write) < msecs_to_jiffies(c->opts.journal_flush_delay) &&
-	     test_bit(JOURNAL_MAY_SKIP_FLUSH, &j->flags))) {
+
+	/*
+	 * If the journal is in an error state - we did an emergency shutdown -
+	 * we prefer to continue doing journal writes. We just mark them as
+	 * noflush so they'll never be used, but they'll still be visible by the
+	 * list_journal tool - this helps in debugging.
+	 *
+	 * There's a caveat: the first journal write after marking the
+	 * superblock dirty must always be a flush write, because on startup
+	 * from a clean shutdown we didn't necessarily read the journal and the
+	 * new journal write might overwrite whatever was in the journal
+	 * previously - we can't leave the journal without any flush writes in
+	 * it.
+	 *
+	 * So if we're in an error state, and we're still starting up, we don't
+	 * write anything at all.
+	 */
+	if (!test_bit(JOURNAL_NEED_FLUSH_WRITE, &j->flags) &&
+	    (bch2_journal_error(j) ||
+	     w->noflush ||
+	     (!w->must_flush &&
+	      (jiffies - j->last_flush_write) < msecs_to_jiffies(c->opts.journal_flush_delay) &&
+	      test_bit(JOURNAL_MAY_SKIP_FLUSH, &j->flags)))) {
 		w->noflush = true;
 		SET_JSET_NO_FLUSH(jset, true);
 		jset->last_seq	= 0;
 		w->last_seq	= 0;
 
 		j->nr_noflush_writes++;
-	} else {
+	} else if (!bch2_journal_error(j)) {
 		j->last_flush_write = jiffies;
 		j->nr_flush_writes++;
+		clear_bit(JOURNAL_NEED_FLUSH_WRITE, &j->flags);
+	} else {
+		spin_unlock(&j->lock);
+		goto err;
 	}
 	spin_unlock(&j->lock);
 

fs/bcachefs/journal_types.h

@@ -141,10 +141,11 @@ enum journal_space_from {
 	journal_space_nr,
 };
 
-enum {
+enum journal_flags {
 	JOURNAL_REPLAY_DONE,
 	JOURNAL_STARTED,
 	JOURNAL_MAY_SKIP_FLUSH,
+	JOURNAL_NEED_FLUSH_WRITE,
 };
 
 #define JOURNAL_WATERMARKS()		\

fs/bcachefs/super.c

@@ -379,6 +379,14 @@ static int __bch2_fs_read_write(struct bch_fs *c, bool early)
 
 	clear_bit(BCH_FS_CLEAN_SHUTDOWN, &c->flags);
 
+	/*
+	 * First journal write must be a flush write: after a clean shutdown we
+	 * don't read the journal, so the first journal write may end up
+	 * overwriting whatever was there previously, and there must always be
+	 * at least one non-flush write in the journal or recovery will fail:
+	 */
+	set_bit(JOURNAL_NEED_FLUSH_WRITE, &c->journal.flags);
+
 	for_each_rw_member(ca, c, i)
 		bch2_dev_allocator_add(c, ca);
 	bch2_recalc_capacity(c);