[PATCH RFC 2/6] btrfs: Store the last device I/O offset

[Michal Rostecki <mrostecki@suse.de>]

From: Michal Rostecki <mrostecki@suse.com>

Add an atomic field which stores the physical offset of the last I/O
operation  scheduled to the device. This information is going to be used
to measure the locality of I/O requests.

Signed-off-by: Michal Rostecki <mrostecki@suse.com>
---
 fs/btrfs/volumes.c | 4 ++++
 fs/btrfs/volumes.h | 1 +
 2 files changed, 5 insertions(+)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index d4f452dcce95..292175206873 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -444,6 +444,7 @@ static struct btrfs_device *__alloc_device(struct btrfs_fs_info *fs_info)
 		kfree(dev);
 		return ERR_PTR(-ENOMEM);
 	}
+	atomic_set(&dev->last_offset, 0);
 
 	return dev;
 }
@@ -6368,11 +6369,13 @@ static void submit_stripe_bio(struct btrfs_bio *bbio, struct bio *bio,
 			      u64 physical, struct btrfs_device *dev)
 {
 	struct btrfs_fs_info *fs_info = bbio->fs_info;
+	u64 length;
 
 	bio->bi_private = bbio;
 	btrfs_io_bio(bio)->device = dev;
 	bio->bi_end_io = btrfs_end_bio;
 	bio->bi_iter.bi_sector = physical >> 9;
+	length = bio->bi_iter.bi_size;
 	btrfs_debug_in_rcu(fs_info,
 	"btrfs_map_bio: rw %d 0x%x, sector=%llu, dev=%lu (%s id %llu), size=%u",
 		bio_op(bio), bio->bi_opf, bio->bi_iter.bi_sector,
@@ -6382,6 +6385,7 @@ static void submit_stripe_bio(struct btrfs_bio *bbio, struct bio *bio,
 
 	btrfs_bio_counter_inc_noblocked(fs_info);
 	percpu_counter_inc(&dev->inflight);
+	atomic_set(&dev->last_offset, physical + length);
 
 	btrfsic_submit_bio(bio);
 }
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index 938c5292250c..6e544317a377 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -146,6 +146,7 @@ struct btrfs_device {
 
 	/* I/O stats for raid1 mirror selection */
 	struct percpu_counter inflight;
+	atomic_t last_offset;
 };
 
 /*
-- 
2.30.0