[PATCH] btrfs: clarify btrfs_ioctl_get_dev_stats padding

[PATCH] btrfs: clarify btrfs_ioctl_get_dev_stats padding

[Hans van Kranenburg]

From: Hans van Kranenburg <hans.van.kranenburg@mendix.com>

In commit c11d2c236cc26 the get_dev_stats ioctl was added.

Shortly thereafter, in commit b27f7c0c150f7, the flags field was added.
However, the calculation for unused padding space was not updated, which
also invalidated the comment.

Clarify what happened to reduce confusion and wasted time for anyone
implementing this.

Signed-off-by: Hans van Kranenburg <hans.van.kranenburg@mendix.com>
---
 include/uapi/linux/btrfs.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/btrfs.h b/include/uapi/linux/btrfs.h
index 7885d79f7515..3ee0678c0a83 100644
--- a/include/uapi/linux/btrfs.h
+++ b/include/uapi/linux/btrfs.h
@@ -665,7 +665,12 @@ struct btrfs_ioctl_get_dev_stats {
 	/* out values: */
 	__u64 values[BTRFS_DEV_STAT_VALUES_MAX];
 
-	__u64 unused[128 - 2 - BTRFS_DEV_STAT_VALUES_MAX]; /* pad to 1k */
+	/*
+	 * This pads the struct to 1032 bytes. It was originally meant to pad to
+	 * 1024 bytes, but when adding the flags field, the padding calculation
+	 * was not adjusted.
+	 */
+	__u64 unused[128 - 2 - BTRFS_DEV_STAT_VALUES_MAX];
 };
 
 #define BTRFS_QUOTA_CTL_ENABLE	1
-- 
2.20.1

Re: [PATCH] btrfs: clarify btrfs_ioctl_get_dev_stats padding

[David Sterba]

On Sat, Aug 03, 2019 at 11:36:34PM +0200, Hans van Kranenburg wrote:
> From: Hans van Kranenburg <hans.van.kranenburg@mendix.com>
> 
> In commit c11d2c236cc26 the get_dev_stats ioctl was added.
> 
> Shortly thereafter, in commit b27f7c0c150f7, the flags field was added.
> However, the calculation for unused padding space was not updated, which
> also invalidated the comment.
> 
> Clarify what happened to reduce confusion and wasted time for anyone
> implementing this.
> 
> Signed-off-by: Hans van Kranenburg <hans.van.kranenburg@mendix.com>

Reviewed-by: David Sterba <dsterba@suse.com>