bcachefs: __bch2_read(): call trans_begin() on every loop iter

perusal of /sys/kernel/debug/bcachefs/*/btree_transaction_stats shows
that the read path has been acculumalating unneeded paths on the reflink
btree, which we don't want.

The solution is to call bch2_trans_begin(), which drops paths not used
on previous loop iteration.

bch2_readahead:
  Max mem used: 0
  Transaction duration:
    count:      194235
                           since mount        recent
    duration of events
      min:                      150 ns
      max:                        9 ms
      total:                    838 ms
      mean:                       4 us          6 us
      stddev:                    34 us          7 us
    time between events
      min:                       10 ns
      max:                       15 h
      mean:                       2 s          12 s
      stddev:                     2 s           3 ms
  Maximum allocated btree paths (193):
    path: idx  2 ref 0:0 P   btree=extents l=0 pos 270943112:392:U32_MAX locks 0
    path: idx  3 ref 1:0   S btree=extents l=0 pos 270943112:24578:U32_MAX locks 1
    path: idx  4 ref 0:0 P   btree=reflink l=0 pos 0:24773509:0 locks 0
    path: idx  5 ref 0:0 P S btree=reflink l=0 pos 0:24773631:0 locks 1
    path: idx  6 ref 0:0 P S btree=reflink l=0 pos 0:24773759:0 locks 1
    path: idx  7 ref 0:0 P S btree=reflink l=0 pos 0:24773887:0 locks 1
    path: idx  8 ref 0:0 P S btree=reflink l=0 pos 0:24774015:0 locks 1
    path: idx  9 ref 0:0 P S btree=reflink l=0 pos 0:24774143:0 locks 1
    path: idx 10 ref 0:0 P S btree=reflink l=0 pos 0:24774271:0 locks 1
<many more reflink paths>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/io_read.c

@@ -1147,34 +1147,27 @@ void __bch2_read(struct bch_fs *c, struct bch_read_bio *rbio,
 	struct btree_iter iter;
 	struct bkey_buf sk;
 	struct bkey_s_c k;
-	u32 snapshot;
 	int ret;
 
 	BUG_ON(flags & BCH_READ_NODECODE);
 
 	bch2_bkey_buf_init(&sk);
-retry:
-	bch2_trans_begin(trans);
-	iter = (struct btree_iter) { NULL };
-
-	ret = bch2_subvolume_get_snapshot(trans, inum.subvol, &snapshot);
-	if (ret)
-		goto err;
-
 	bch2_trans_iter_init(trans, &iter, BTREE_ID_extents,
-			     SPOS(inum.inum, bvec_iter.bi_sector, snapshot),
+			     POS(inum.inum, bvec_iter.bi_sector),
 			     BTREE_ITER_slots);
+
 	while (1) {
 		unsigned bytes, sectors, offset_into_extent;
 		enum btree_id data_btree = BTREE_ID_extents;
 
-		/*
-		 * read_extent -> io_time_reset may cause a transaction restart
-		 * without returning an error, we need to check for that here:
-		 */
-		ret = bch2_trans_relock(trans);
+		bch2_trans_begin(trans);
+
+		u32 snapshot;
+		ret = bch2_subvolume_get_snapshot(trans, inum.subvol, &snapshot);
 		if (ret)
-			break;
+			goto err;
+
+		bch2_btree_iter_set_snapshot(&iter, snapshot);
 
 		bch2_btree_iter_set_pos(&iter,
 				POS(inum.inum, bvec_iter.bi_sector));
@@ -1182,7 +1175,7 @@ void __bch2_read(struct bch_fs *c, struct bch_read_bio *rbio,
 		k = bch2_btree_iter_peek_slot(&iter);
 		ret = bkey_err(k);
 		if (ret)
-			break;
+			goto err;
 
 		offset_into_extent = iter.pos.offset -
 			bkey_start_offset(k.k);
@@ -1193,7 +1186,7 @@ void __bch2_read(struct bch_fs *c, struct bch_read_bio *rbio,
 		ret = bch2_read_indirect_extent(trans, &data_btree,
 					&offset_into_extent, &sk);
 		if (ret)
-			break;
+			goto err;
 
 		k = bkey_i_to_s_c(sk.k);
 
@@ -1213,7 +1206,7 @@ void __bch2_read(struct bch_fs *c, struct bch_read_bio *rbio,
 					 data_btree, k,
 					 offset_into_extent, failed, flags);
 		if (ret)
-			break;
+			goto err;
 
 		if (flags & BCH_READ_LAST_FRAGMENT)
 			break;
@@ -1223,16 +1216,16 @@ void __bch2_read(struct bch_fs *c, struct bch_read_bio *rbio,
 
 		ret = btree_trans_too_many_iters(trans);
 		if (ret)
+			goto err;
+err:
+		if (ret &&
+		    !bch2_err_matches(ret, BCH_ERR_transaction_restart) &&
+		    ret != READ_RETRY &&
+		    ret != READ_RETRY_AVOID)
 			break;
 	}
-err:
-	bch2_trans_iter_exit(trans, &iter);
-
-	if (bch2_err_matches(ret, BCH_ERR_transaction_restart) ||
-	    ret == READ_RETRY ||
-	    ret == READ_RETRY_AVOID)
-		goto retry;
 
+	bch2_trans_iter_exit(trans, &iter);
 	bch2_trans_put(trans);
 	bch2_bkey_buf_exit(&sk, c);