btrfs: shrink the size of btrfs_device

There are two main causes of holes inside btrfs_device:

- The single bytes member of last_flush_error
  Not only it's a single byte member, but we never really care about the
  exact error number.

- The @devt member
  Which is placed between two u64 members.

Shrink the size of btrfs_device by:

- Use a single bit flag for flush error
  Use BTRFS_DEV_STATE_FLUSH_FAILED so that we no longer need that
  dedicated member.

- Move @devt to the hole after dev_stat_values[]

This reduces the size of btrfs_device from 528 to exact 512 bytes for
x86_64.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

3 files changed, 12 insertions, 11 deletions

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 9bb5d65219a7..faa1c2c20ecd 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -3834,7 +3834,7 @@ static void write_dev_flush(struct btrfs_device *device)
 {
 	struct bio *bio = &device->flush_bio;
 
-	device->last_flush_error = BLK_STS_OK;
+	clear_bit(BTRFS_DEV_STATE_FLUSH_FAILED, &device->dev_state);
 
 	bio_init(bio, device->bdev, NULL, 0,
 		 REQ_OP_WRITE | REQ_SYNC | REQ_PREFLUSH);
@@ -3859,7 +3859,7 @@ static bool wait_dev_flush(struct btrfs_device *device)
 	wait_for_completion_io(&device->flush_wait);
 
 	if (bio->bi_status) {
-		device->last_flush_error = bio->bi_status;
+		set_bit(BTRFS_DEV_STATE_FLUSH_FAILED, &device->dev_state);
 		btrfs_dev_stat_inc_and_print(device, BTRFS_DEV_STAT_FLUSH_ERRS);
 		return true;
 	}
@@ -3909,7 +3909,7 @@ static int barrier_all_devices(struct btrfs_fs_info *info)
 	}
 
 	/*
-	 * Checks last_flush_error of disks in order to determine the device
+	 * Checks flush failure of disks in order to determine the device
 	 * state.
 	 */
 	if (unlikely(errors_wait && !btrfs_check_rw_degradable(info, NULL)))
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index a541cd30c6b8..844657f23e7d 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -1169,7 +1169,7 @@ static void btrfs_close_one_device(struct btrfs_device *device)
 	 * any transaction and set the error state, guaranteeing no commits of
 	 * unsafe super blocks.
 	 */
-	device->last_flush_error = 0;
+	clear_bit(BTRFS_DEV_STATE_FLUSH_FAILED, &device->dev_state);
 
 	/* Verify the device is back in a pristine state  */
 	WARN_ON(test_bit(BTRFS_DEV_STATE_FLUSH_SENT, &device->dev_state));
@@ -7375,7 +7375,7 @@ bool btrfs_check_rw_degradable(struct btrfs_fs_info *fs_info,
 
 			if (!dev || !dev->bdev ||
 			    test_bit(BTRFS_DEV_STATE_MISSING, &dev->dev_state) ||
-			    dev->last_flush_error)
+			    test_bit(BTRFS_DEV_STATE_FLUSH_FAILED, &dev->dev_state))
 				missing++;
 			else if (failing_dev && failing_dev == dev)
 				missing++;
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index 262526657cdf..59347a4bb185 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -99,6 +99,7 @@ enum btrfs_raid_types {
 #define BTRFS_DEV_STATE_REPLACE_TGT	(3)
 #define BTRFS_DEV_STATE_FLUSH_SENT	(4)
 #define BTRFS_DEV_STATE_NO_READA	(5)
+#define BTRFS_DEV_STATE_FLUSH_FAILED	(6)
 
 /* Set when the device item is found in chunk tree, used to catch unexpected registered device. */
 #define BTRFS_DEV_STATE_ITEM_FOUND	(7)
@@ -125,13 +126,7 @@ struct btrfs_device {
 
 	struct btrfs_zoned_device_info *zone_info;
 
-	/*
-	 * Device's major-minor number. Must be set even if the device is not
-	 * opened (bdev == NULL), unless the device is missing.
-	 */
-	dev_t devt;
 	unsigned long dev_state;
-	blk_status_t last_flush_error;
 
 #ifdef __BTRFS_NEED_DEVICE_DATA_ORDERED
 	seqcount_t data_seqcount;
@@ -195,6 +190,12 @@ struct btrfs_device {
 	atomic_t dev_stats_ccnt;
 	atomic_t dev_stat_values[BTRFS_DEV_STAT_VALUES_MAX];
 
+	/*
+	 * Device's major-minor number. Must be set even if the device is not
+	 * opened (bdev == NULL), unless the device is missing.
+	 */
+	dev_t devt;
+
 	struct extent_io_tree alloc_state;
 
 	struct completion kobj_unregister;