btrfs: stop allocation a btrfs_io_context for simple I/O

The I/O context structure is only used to pass the btrfs_device to
the end I/O handler for I/Os that go to a single device.

Stop allocating the I/O context for these cases by passing the optional
btrfs_io_stripe argument to __btrfs_map_block to query the mapping
information and then using a fast path submission and I/O completion
handler.  As the old btrfs_io_context based I/O submission path is
only used for mirrored writes, rename the functions to make that
clear and stop setting the btrfs_bio device and mirror_num field
that is only used for reads.
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Tested-by: Nikolay Borisov <nborisov@suse.com>
Tested-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -6706,11 +6706,12 @@ static void btrfs_log_dev_io_error(struct bio *bio, struct btrfs_device *dev)
 		btrfs_dev_stat_inc_and_print(dev, BTRFS_DEV_STAT_FLUSH_ERRS);
 }
 
-static struct workqueue_struct *btrfs_end_io_wq(struct btrfs_io_context *bioc)
+static struct workqueue_struct *btrfs_end_io_wq(struct btrfs_fs_info *fs_info,
+						struct bio *bio)
 {
-	if (bioc->orig_bio->bi_opf & REQ_META)
-		return bioc->fs_info->endio_meta_workers;
-	return bioc->fs_info->endio_workers;
+	if (bio->bi_opf & REQ_META)
+		return fs_info->endio_meta_workers;
+	return fs_info->endio_workers;
 }
 
 static void btrfs_end_bio_work(struct work_struct *work)
@@ -6721,6 +6722,24 @@ static void btrfs_end_bio_work(struct work_struct *work)
 	bbio->end_io(bbio);
 }
 
+static void btrfs_simple_end_io(struct bio *bio)
+{
+	struct btrfs_fs_info *fs_info = bio->bi_private;
+	struct btrfs_bio *bbio = btrfs_bio(bio);
+
+	btrfs_bio_counter_dec(fs_info);
+
+	if (bio->bi_status)
+		btrfs_log_dev_io_error(bio, bbio->device);
+
+	if (bio_op(bio) == REQ_OP_READ) {
+		INIT_WORK(&bbio->end_io_work, btrfs_end_bio_work);
+		queue_work(btrfs_end_io_wq(fs_info, bio), &bbio->end_io_work);
+	} else {
+		bbio->end_io(bbio);
+	}
+}
+
 static void btrfs_raid56_end_io(struct bio *bio)
 {
 	struct btrfs_io_context *bioc = bio->bi_private;
@@ -6733,7 +6752,7 @@ static void btrfs_raid56_end_io(struct bio *bio)
 	btrfs_put_bioc(bioc);
 }
 
-static void btrfs_end_bio(struct bio *bio)
+static void btrfs_orig_write_end_io(struct bio *bio)
 {
 	struct btrfs_io_stripe *stripe = bio->bi_private;
 	struct btrfs_io_context *bioc = stripe->bioc;
@@ -6746,8 +6765,6 @@ static void btrfs_end_bio(struct bio *bio)
 		btrfs_log_dev_io_error(bio, stripe->dev);
 	}
 
-	bbio->mirror_num = bioc->mirror_num;
-
 	/*
 	 * Only send an error to the higher layers if it is beyond the tolerance
 	 * threshold.
@@ -6757,13 +6774,7 @@ static void btrfs_end_bio(struct bio *bio)
 	else
 		bio->bi_status = BLK_STS_OK;
 
-	if (btrfs_op(bio) == BTRFS_MAP_READ) {
-		INIT_WORK(&bbio->end_io_work, btrfs_end_bio_work);
-		queue_work(btrfs_end_io_wq(bioc), &bbio->end_io_work);
-	} else {
-		bbio->end_io(bbio);
-	}
-
+	bbio->end_io(bbio);
 	btrfs_put_bioc(bioc);
 }
 
@@ -6820,15 +6831,16 @@ static void btrfs_submit_dev_bio(struct btrfs_device *dev, struct bio *bio)
 	submit_bio(bio);
 }
 
-static void submit_stripe_bio(struct btrfs_io_context *bioc, int dev_nr)
+static void btrfs_submit_mirrored_bio(struct btrfs_io_context *bioc, int dev_nr)
 {
 	struct bio *orig_bio = bioc->orig_bio, *bio;
 
+	ASSERT(bio_op(orig_bio) != REQ_OP_READ);
+
 	/* Reuse the bio embedded into the btrfs_bio for the last mirror */
 	if (dev_nr == bioc->num_stripes - 1) {
 		bio = orig_bio;
-		btrfs_bio(bio)->device = bioc->stripes[dev_nr].dev;
-		bio->bi_end_io = btrfs_end_bio;
+		bio->bi_end_io = btrfs_orig_write_end_io;
 	} else {
 		bio = bio_alloc_clone(NULL, orig_bio, GFP_NOFS, &fs_bio_set);
 		bio_inc_remaining(orig_bio);
@@ -6846,34 +6858,19 @@ void btrfs_submit_bio(struct btrfs_fs_info *fs_info, struct bio *bio, int mirror
 	u64 logical = bio->bi_iter.bi_sector << 9;
 	u64 length = bio->bi_iter.bi_size;
 	u64 map_length = length;
-	int ret;
-	int dev_nr;
-	int total_devs;
 	struct btrfs_io_context *bioc = NULL;
+	struct btrfs_io_stripe smap;
+	int ret;
 
 	btrfs_bio_counter_inc_blocked(fs_info);
 	ret = __btrfs_map_block(fs_info, btrfs_op(bio), logical, &map_length,
-				&bioc, NULL, &mirror_num, 1);
+				&bioc, &smap, &mirror_num, 1);
 	if (ret) {
 		btrfs_bio_counter_dec(fs_info);
 		btrfs_bio_end_io(btrfs_bio(bio), errno_to_blk_status(ret));
 		return;
 	}
 
-	total_devs = bioc->num_stripes;
-	bioc->orig_bio = bio;
-
-	if ((bioc->map_type & BTRFS_BLOCK_GROUP_RAID56_MASK) &&
-	    ((btrfs_op(bio) == BTRFS_MAP_WRITE) || (mirror_num > 1))) {
-		bio->bi_private = bioc;
-		bio->bi_end_io = btrfs_raid56_end_io;
-		if (btrfs_op(bio) == BTRFS_MAP_WRITE)
-			raid56_parity_write(bio, bioc);
-		else
-			raid56_parity_recover(bio, bioc, mirror_num);
-		return;
-	}
-
 	if (map_length < length) {
 		btrfs_crit(fs_info,
 			   "mapping failed logical %llu bio len %llu len %llu",
@@ -6881,8 +6878,31 @@ void btrfs_submit_bio(struct btrfs_fs_info *fs_info, struct bio *bio, int mirror
 		BUG();
 	}
 
-	for (dev_nr = 0; dev_nr < total_devs; dev_nr++)
-		submit_stripe_bio(bioc, dev_nr);
+	if (!bioc) {
+		/* Single mirror read/write fast path */
+		btrfs_bio(bio)->mirror_num = mirror_num;
+		btrfs_bio(bio)->device = smap.dev;
+		bio->bi_iter.bi_sector = smap.physical >> SECTOR_SHIFT;
+		bio->bi_private = fs_info;
+		bio->bi_end_io = btrfs_simple_end_io;
+		btrfs_submit_dev_bio(smap.dev, bio);
+	} else if (bioc->map_type & BTRFS_BLOCK_GROUP_RAID56_MASK) {
+		/* Parity RAID write or read recovery */
+		bio->bi_private = bioc;
+		bio->bi_end_io = btrfs_raid56_end_io;
+		if (bio_op(bio) == REQ_OP_READ)
+			raid56_parity_recover(bio, bioc, mirror_num);
+		else
+			raid56_parity_write(bio, bioc);
+	} else {
+		/* Write to multiple mirrors */
+		int total_devs = bioc->num_stripes;
+		int dev_nr;
+
+		bioc->orig_bio = bio;
+		for (dev_nr = 0; dev_nr < total_devs; dev_nr++)
+			btrfs_submit_mirrored_bio(bioc, dev_nr);
+	}
 }
 
 static bool dev_args_match_fs_devices(const struct btrfs_dev_lookup_args *args,