bcachefs: Avoid atomics in write fast path

This adds some horrible hacks, but the atomic ops for closures were
getting to be a pretty expensive part of the write path. We don't want
to rip out closures entirely from the write path, because they're used
for e.g. waiting on the allocator, or waiting on the journal flush, and
that stuff would get really ugly without closures.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/fs-io.c

@@ -54,7 +54,7 @@ struct bch_writepage_io {
 };
 
 struct dio_write {
-	struct closure			cl;
+	struct completion		done;
 	struct kiocb			*req;
 	struct mm_struct		*mm;
 	unsigned			loop:1,
@@ -1755,8 +1755,6 @@ static noinline int bch2_dio_write_copy_iov(struct dio_write *dio)
 	return 0;
 }
 
-static void bch2_dio_write_loop_async(struct closure *);
-
 static long bch2_dio_write_loop(struct dio_write *dio)
 {
 	bool kthread = (current->flags & PF_KTHREAD) != 0;
@@ -1830,23 +1828,20 @@ static long bch2_dio_write_loop(struct dio_write *dio)
 
 		task_io_account_write(bio->bi_iter.bi_size);
 
-		closure_call(&dio->op.cl, bch2_write, NULL, &dio->cl);
-
 		if (!dio->sync && !dio->loop && dio->iter.count) {
 			if (bch2_dio_write_copy_iov(dio)) {
-				dio->op.error = -ENOMEM;
-				goto err_wait_io;
+				dio->sync = true;
+				goto do_io;
 			}
 		}
-err_wait_io:
+do_io:
 		dio->loop = true;
+		closure_call(&dio->op.cl, bch2_write, NULL, NULL);
 
-		if (!dio->sync) {
-			continue_at(&dio->cl, bch2_dio_write_loop_async, NULL);
+		if (dio->sync)
+			wait_for_completion(&dio->done);
+		else
 			return -EIOCBQUEUED;
-		}
-
-		closure_sync(&dio->cl);
 loop:
 		i_sectors_acct(c, inode, &dio->quota_res,
 			       dio->op.i_sectors_delta);
@@ -1863,7 +1858,9 @@ static long bch2_dio_write_loop(struct dio_write *dio)
 			put_page(bv->bv_page);
 		if (!dio->iter.count || dio->op.error)
 			break;
+
 		bio_reset(bio, NULL, REQ_OP_WRITE);
+		reinit_completion(&dio->done);
 	}
 
 	ret = dio->op.error ?: ((long) dio->op.written << 9);
@@ -1875,8 +1872,6 @@ static long bch2_dio_write_loop(struct dio_write *dio)
 	if (dio->free_iov)
 		kfree(dio->iter.__iov);
 
-	closure_debug_destroy(&dio->cl);
-
 	sync = dio->sync;
 	bio_put(bio);
 
@@ -1890,11 +1885,14 @@ static long bch2_dio_write_loop(struct dio_write *dio)
 	return ret;
 }
 
-static void bch2_dio_write_loop_async(struct closure *cl)
+static void bch2_dio_write_loop_async(struct bch_write_op *op)
 {
-	struct dio_write *dio = container_of(cl, struct dio_write, cl);
+	struct dio_write *dio = container_of(op, struct dio_write, op);
 
-	bch2_dio_write_loop(dio);
+	if (dio->sync)
+		complete(&dio->done);
+	else
+		bch2_dio_write_loop(dio);
 }
 
 static noinline
@@ -1922,7 +1920,7 @@ ssize_t bch2_direct_write(struct kiocb *req, struct iov_iter *iter)
 			       GFP_KERNEL,
 			       &c->dio_write_bioset);
 	dio = container_of(bio, struct dio_write, op.wbio.bio);
-	closure_init(&dio->cl, NULL);
+	init_completion(&dio->done);
 	dio->req		= req;
 	dio->mm			= current->mm;
 	dio->loop		= false;
@@ -1933,6 +1931,7 @@ ssize_t bch2_direct_write(struct kiocb *req, struct iov_iter *iter)
 	dio->iter		= *iter;
 
 	bch2_write_op_init(&dio->op, c, opts);
+	dio->op.end_io		= bch2_dio_write_loop_async;
 	dio->op.target		= opts.foreground_target;
 	op_journal_seq_set(&dio->op, &inode->ei_journal_seq);
 	dio->op.write_point	= writepoint_hashed((unsigned long) current);
@@ -1962,7 +1961,6 @@ ssize_t bch2_direct_write(struct kiocb *req, struct iov_iter *iter)
 err:
 	bch2_disk_reservation_put(c, &dio->op.res);
 	bch2_quota_reservation_put(c, inode, &dio->quota_res);
-	closure_debug_destroy(&dio->cl);
 	bio_put(bio);
 	return ret;
 }

fs/bcachefs/io.c

@@ -513,7 +513,12 @@ static void bch2_write_done(struct closure *cl)
 
 	bch2_time_stats_update(&c->times[BCH_TIME_data_write], op->start_time);
 
-	closure_return(cl);
+	if (op->end_io)
+		op->end_io(op);
+	if (cl->parent)
+		closure_return(cl);
+	else
+		closure_debug_destroy(cl);
 }
 
 /**
@@ -622,8 +627,10 @@ static void bch2_write_endio(struct bio *bio)
 
 	if (parent)
 		bio_endio(&parent->bio);
-	else
+	else if (!(op->flags & BCH_WRITE_SKIP_CLOSURE_PUT))
 		closure_put(cl);
+	else
+		continue_at_nobarrier(cl, bch2_write_index, index_update_wq(op));
 }
 
 static void init_append_extent(struct bch_write_op *op,
@@ -828,15 +835,14 @@ static enum prep_encoded_ret {
 	return PREP_ENCODED_OK;
 }
 
-static int bch2_write_extent(struct bch_write_op *op, struct write_point *wp)
+static int bch2_write_extent(struct bch_write_op *op, struct write_point *wp,
+			     struct bio **_dst)
 {
 	struct bch_fs *c = op->c;
 	struct bio *src = &op->wbio.bio, *dst = src;
 	struct bvec_iter saved_iter;
-	struct bkey_i *key_to_write;
 	void *ec_buf;
-	unsigned key_to_write_offset = op->insert_keys.top_p -
-		op->insert_keys.keys_p;
+	struct bpos ec_pos = op->pos;
 	unsigned total_output = 0, total_input = 0;
 	bool bounce = false;
 	bool page_alloc_failed = false;
@@ -855,6 +861,7 @@ static int bch2_write_extent(struct bch_write_op *op, struct write_point *wp)
 	case PREP_ENCODED_CHECKSUM_ERR:
 		goto csum_err;
 	case PREP_ENCODED_DO_WRITE:
+		/* XXX look for bug here */
 		if (ec_buf) {
 			dst = bch2_write_bio_alloc(c, wp, src,
 						   &page_alloc_failed,
@@ -1004,31 +1011,15 @@ static int bch2_write_extent(struct bch_write_op *op, struct write_point *wp)
 	dst->bi_iter.bi_size = total_output;
 do_write:
 	/* might have done a realloc... */
+	bch2_ec_add_backpointer(c, wp, ec_pos, total_input >> 9);
 
-	key_to_write = (void *) (op->insert_keys.keys_p + key_to_write_offset);
-
-	bch2_ec_add_backpointer(c, wp,
-				bkey_start_pos(&key_to_write->k),
-				total_input >> 9);
-
-	bch2_alloc_sectors_done(c, wp);
-
-	dst->bi_end_io	= bch2_write_endio;
-	dst->bi_private	= &op->cl;
-	dst->bi_opf	= REQ_OP_WRITE;
-
-	closure_get(dst->bi_private);
-
-	bch2_submit_wbio_replicas(to_wbio(dst), c, BCH_DATA_USER,
-				  key_to_write);
+	*_dst = dst;
 	return more;
 csum_err:
 	bch_err(c, "error verifying existing checksum while "
 		"rewriting existing data (memory corruption?)");
 	ret = -EIO;
 err:
-	bch2_alloc_sectors_done(c, wp);
-
 	if (to_wbio(dst)->bounce)
 		bch2_bio_free_pages_pool(c, dst);
 	if (to_wbio(dst)->put_bio)
@@ -1042,11 +1033,17 @@ static void __bch2_write(struct closure *cl)
 	struct bch_write_op *op = container_of(cl, struct bch_write_op, cl);
 	struct bch_fs *c = op->c;
 	struct write_point *wp;
+	struct bio *bio;
+	bool skip_put = true;
 	int ret;
 again:
 	memset(&op->failed, 0, sizeof(op->failed));
 
 	do {
+		struct bkey_i *key_to_write;
+		unsigned key_to_write_offset = op->insert_keys.top_p -
+			op->insert_keys.keys_p;
+
 		/* +1 for possible cache device: */
 		if (op->open_buckets.nr + op->nr_replicas + 1 >
 		    ARRAY_SIZE(op->open_buckets.v))
@@ -1080,21 +1077,38 @@ static void __bch2_write(struct closure *cl)
 		}
 
 		bch2_open_bucket_get(c, wp, &op->open_buckets);
-
-		ret = bch2_write_extent(op, wp);
+		ret = bch2_write_extent(op, wp, &bio);
+		bch2_alloc_sectors_done(c, wp);
 
 		if (ret < 0)
 			goto err;
+
+		if (ret)
+			skip_put = false;
+
+		bio->bi_end_io	= bch2_write_endio;
+		bio->bi_private	= &op->cl;
+		bio->bi_opf	= REQ_OP_WRITE;
+
+		if (!skip_put)
+			closure_get(bio->bi_private);
+		else
+			op->flags |= BCH_WRITE_SKIP_CLOSURE_PUT;
+
+		key_to_write = (void *) (op->insert_keys.keys_p +
+					 key_to_write_offset);
+
+		bch2_submit_wbio_replicas(to_wbio(bio), c, BCH_DATA_USER,
+					  key_to_write);
 	} while (ret);
 
-	continue_at(cl, bch2_write_index, index_update_wq(op));
+	if (!skip_put)
+		continue_at(cl, bch2_write_index, index_update_wq(op));
 	return;
 err:
 	op->error = ret;
 
-	continue_at(cl, !bch2_keylist_empty(&op->insert_keys)
-		    ? bch2_write_index
-		    : bch2_write_done, index_update_wq(op));
+	continue_at(cl, bch2_write_index, index_update_wq(op));
 	return;
 flush_io:
 	closure_sync(cl);

fs/bcachefs/io.h

@@ -37,6 +37,7 @@ enum bch_write_flags {
 
 	/* Internal: */
 	BCH_WRITE_JOURNAL_SEQ_PTR	= (1 << 8),
+	BCH_WRITE_SKIP_CLOSURE_PUT	= (1 << 9),
 };
 
 static inline u64 *op_journal_seq(struct bch_write_op *op)
@@ -71,6 +72,7 @@ static inline void bch2_write_op_init(struct bch_write_op *op, struct bch_fs *c,
 				      struct bch_io_opts opts)
 {
 	op->c			= c;
+	op->end_io		= NULL;
 	op->flags		= 0;
 	op->written		= 0;
 	op->error		= 0;

fs/bcachefs/io_types.h

@@ -95,6 +95,7 @@ struct bch_write_bio {
 struct bch_write_op {
 	struct closure		cl;
 	struct bch_fs		*c;
+	void			(*end_io)(struct bch_write_op *);
 	u64			start_time;
 
 	unsigned		written; /* sectors */