bcachefs: Fix livelock in journal_entry_open()

When the journal is low on space, we might do discards from
journal_res_get() -> journal_entry_open().

Make sure we set j->can_discard correctly, so that if we're low on space
but not because discards aren't keeping up we don't livelock.

Fixes: 8e4d280 ("bcachefs: Don't aggressively discard the journal")
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Author: Kent Overstreet

fs/bcachefs/journal_reclaim.c

@@ -17,6 +17,8 @@
 #include <linux/kthread.h>
 #include <linux/sched/mm.h>
 
+static bool __should_discard_bucket(struct journal *, struct journal_device *);
+
 /* Free space calculations: */
 
 static unsigned journal_space_from(struct journal_device *ja,
@@ -203,8 +205,7 @@ void bch2_journal_space_available(struct journal *j)
 		       ja->bucket_seq[ja->dirty_idx_ondisk] < j->last_seq_ondisk)
 			ja->dirty_idx_ondisk = (ja->dirty_idx_ondisk + 1) % ja->nr;
 
-		if (ja->discard_idx != ja->dirty_idx_ondisk)
-			can_discard = true;
+		can_discard |= __should_discard_bucket(j, ja);
 
 		max_entry_size = min_t(unsigned, max_entry_size, ca->mi.bucket_size);
 		nr_online++;
@@ -264,13 +265,19 @@ void bch2_journal_space_available(struct journal *j)
 
 /* Discards - last part of journal reclaim: */
 
-static bool should_discard_bucket(struct journal *j, struct journal_device *ja)
+static bool __should_discard_bucket(struct journal *j, struct journal_device *ja)
 {
-	spin_lock(&j->lock);
 	unsigned min_free = max(4, ja->nr / 8);
 
-	bool ret = bch2_journal_dev_buckets_available(j, ja, journal_space_discarded) < min_free &&
+	return bch2_journal_dev_buckets_available(j, ja, journal_space_discarded) <
+		min_free &&
 		ja->discard_idx != ja->dirty_idx_ondisk;
+}
+
+static bool should_discard_bucket(struct journal *j, struct journal_device *ja)
+{
+	spin_lock(&j->lock);
+	bool ret = __should_discard_bucket(j, ja);
 	spin_unlock(&j->lock);
 
 	return ret;