bcachefs: Fix for journal getting stuck

The journal can get stuck if we need to get a journal reservation for
something we have a pre-reservation for, but aren't able to reclaim
space, or if the pin fifo is full - it's impractical to resize the pin
fifo at runtime.

Previously, we reserved 8 entries in the pin fifo for pre-reservations,
but that seems small - we're seeing the journal occasionally get stuck.
Let's reserve a quarter of it.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/journal.c

@@ -416,7 +416,7 @@ static int __journal_res_get(struct journal *j, struct journal_res *res,
 	    (flags & JOURNAL_RES_GET_RESERVED)) {
 		char *journal_debug_buf = kmalloc(4096, GFP_ATOMIC);
 
-		bch_err(c, "Journal stuck!");
+		bch_err(c, "Journal stuck! Hava a pre-reservation but journal full");
 		if (journal_debug_buf) {
 			bch2_journal_debug_to_text(&_PBUF(journal_debug_buf, 4096), j);
 			bch_err(c, "%s", journal_debug_buf);

fs/bcachefs/journal.h

@@ -372,7 +372,7 @@ static inline bool journal_check_may_get_unreserved(struct journal *j)
 {
 	union journal_preres_state s = READ_ONCE(j->prereserved);
 	bool ret = s.reserved < s.remaining &&
-		fifo_free(&j->pin) > 8;
+		fifo_free(&j->pin) > j->pin.size / 4;
 
 	lockdep_assert_held(&j->lock);