linux-kernel - Re: [PATCH v2 09/15] blktrace: add definitions for struct blk_io

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Message-ID: <20251003073111.GD12624@lst.de>
Date: Fri, 3 Oct 2025 09:31:11 +0200
From: Christoph Hellwig <hch@....de>
To: Johannes Thumshirn <johannes.thumshirn@....com>
Cc: Jens Axboe <axboe@...nel.dk>, Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org, linux-btrace@...r.kernel.org,
	John Garry <john.g.garry@...cle.com>,
	Hannes Reinecke <hare@...e.de>, Damien Le Moal <dlemoal@...nel.org>,
	Christoph Hellwig <hch@....de>, Naohiro Aota <naohiro.aota@....com>,
	Shinichiro Kawasaki <shinichiro.kawasaki@....com>,
	Chaitanya Kulkarni <chaitanyak@...dia.com>,
	"Martin K . Petersen" <martin.petersen@...cle.com>
Subject: Re: [PATCH v2 09/15] blktrace: add definitions for struct
 blk_io_trace2

On Thu, Sep 25, 2025 at 05:02:25PM +0200, Johannes Thumshirn wrote:
> Add definitions for the extended version of the blktrace protocol using a
> wider action type to be able to record new actions in the kernel.
> 
> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@....com>
> ---
>  include/uapi/linux/blktrace_api.h | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/include/uapi/linux/blktrace_api.h b/include/uapi/linux/blktrace_api.h
> index a3b1f35ac026..d58ef484de49 100644
> --- a/include/uapi/linux/blktrace_api.h
> +++ b/include/uapi/linux/blktrace_api.h
> @@ -94,6 +94,7 @@ enum blktrace_notify {
>  
>  #define BLK_IO_TRACE_MAGIC	0x65617400
>  #define BLK_IO_TRACE_VERSION	0x07
> +#define BLK_IO_TRACE2_VERSION	0x08
>  
>  /*
>   * The trace itself
> @@ -113,6 +114,21 @@ struct blk_io_trace {
>  	/* cgroup id will be stored here if exists */
>  };
>  
> +struct blk_io_trace2 {
> +	__u32 magic;		/* MAGIC << 8 | BLK_IO_TRACE2_VERSION */
> +	__u32 sequence;		/* event number */
> +	__u64 time;		/* in nanoseconds */
> +	__u64 sector;		/* disk offset */
> +	__u32 bytes;		/* transfer length */
> +	__u32 pid;		/* who did it */
> +	__u64 action;		/* what happened */
> +	__u32 device;		/* device number */
> +	__u32 cpu;		/* on what cpu did it happen */
> +	__u16 error;		/* completion error */
> +	__u16 pdu_len;		/* length of data after this trace */
> +	__u8 pad[8];

This will cause mismatching sizes between x86_32 and other
architectures because the size is not 8-byte aligned.  You'll need
to add or remove 4 bytes of padding to ensure that.

I really wish we could have good helpers to check for that.