Re: [PATCH 2/4] block: use the same failfast bits for bio and request

[Boaz Harrosh <bharrosh@panasas.com>]

On 07/03/2009 11:48 AM, Tejun Heo wrote:
> bio and request use the same set of failfast bits.  This patch makes
> the following changes to simplify things.
> 
> * enumify BIO_RW* bits and reorder bits such that BIOS_RW_FAILFAST_*
>   bits coincide with __REQ_FAILFAST_* bits.
> 
> * The above pushes BIO_RW_AHEAD out of sync with __REQ_FAILFAST_DEV
>   but the matching is useless anyway.  init_request_from_bio() is
>   responsible for setting FAILFAST bits on FS requests and non-FS
>   requests never use BIO_RW_AHEAD.  Drop the code and comment from
>   blk_rq_bio_prep().
> 
> * Define REQ_FAILFAST_MASK which is OR of all FAILFAST bits and
>   simplify FAILFAST flags handling in init_request_from_bio().
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Cc: Jens Axboe <axboe@kernel.dk>

Hi Tejun.

Thanks for doing this, it has been neglected for a long time.
However, it will happen again, I don't like these implicit matches
which are not enforced, They get to drift away. There are several ways
to make sure two sets of enums stay in sync. (I'll have a try at it
tomorrow. if you want). 

> ---
>  block/blk-core.c       |   19 +++++++------------
>  include/linux/bio.h    |   43 +++++++++++++++++++++++--------------------
>  include/linux/blkdev.h |    4 ++++
>  3 files changed, 34 insertions(+), 32 deletions(-)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 8f4b9e0..cd3b265 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -1119,17 +1119,13 @@ void init_request_from_bio(struct request *req, struct bio *bio)
>  	req->cmd_type = REQ_TYPE_FS;
>  
>  	/*
> -	 * inherit FAILFAST from bio (for read-ahead, and explicit FAILFAST)
> +	 * Inherit FAILFAST from bio (for read-ahead, and explicit
> +	 * FAILFAST).  FAILFAST flags are identical for req and bio.
>  	 */
>  	if (bio_rw_ahead(bio))
> -		req->cmd_flags |= (REQ_FAILFAST_DEV | REQ_FAILFAST_TRANSPORT |
> -				   REQ_FAILFAST_DRIVER);
> -	if (bio_failfast_dev(bio))
> -		req->cmd_flags |= REQ_FAILFAST_DEV;
> -	if (bio_failfast_transport(bio))
> -		req->cmd_flags |= REQ_FAILFAST_TRANSPORT;
> -	if (bio_failfast_driver(bio))
> -		req->cmd_flags |= REQ_FAILFAST_DRIVER;
> +		req->cmd_flags |= REQ_FAILFAST_MASK;
> +	else
> +		req->cmd_flags |= bio->bi_rw & REQ_FAILFAST_MASK;
>  
>  	if (unlikely(bio_discard(bio))) {
>  		req->cmd_flags |= REQ_DISCARD;
> @@ -2247,9 +2243,8 @@ EXPORT_SYMBOL_GPL(__blk_end_request_cur);
>  void blk_rq_bio_prep(struct request_queue *q, struct request *rq,
>  		     struct bio *bio)
>  {
> -	/* Bit 0 (R/W) is identical in rq->cmd_flags and bio->bi_rw, and
> -	   we want BIO_RW_AHEAD (bit 1) to imply REQ_FAILFAST (bit 1). */
> -	rq->cmd_flags |= (bio->bi_rw & 3);
> +	/* Bit 0 (R/W) is identical in rq->cmd_flags and bio->bi_rw */
> +	rq->cmd_flags |= bio->bi_rw & REQ_RW;
>  
>  	if (bio_has_data(bio)) {
>  		rq->nr_phys_segments = bio_phys_segments(q, bio);
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index 2892b71..a299ed3 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -142,37 +142,40 @@ struct bio {
>   *
>   * bit 0 -- data direction
>   *	If not set, bio is a read from device. If set, it's a write to device.
> - * bit 1 -- rw-ahead when set
> - * bit 2 -- barrier
> + * bit 1 -- fail fast device errors
> + * bit 2 -- fail fast transport errors
> + * bit 3 -- fail fast driver errors
> + * bit 4 -- rw-ahead when set
> + * bit 5 -- barrier

Please kill all these evil bit 1, bit 2 ,bit n comments. The ways we
invent to torture ourselfs...

Just move all the comments to the enums declarations below. And be done
with it, also for the next time.

>   *	Insert a serialization point in the IO queue, forcing previously
>   *	submitted IO to be completed before this one is issued.
> - * bit 3 -- synchronous I/O hint.
> - * bit 4 -- Unplug the device immediately after submitting this bio.
> - * bit 5 -- metadata request
> + * bit 6 -- synchronous I/O hint.
> + * bit 7 -- Unplug the device immediately after submitting this bio.
> + * bit 8 -- metadata request
>   *	Used for tracing to differentiate metadata and data IO. May also
>   *	get some preferential treatment in the IO scheduler
> - * bit 6 -- discard sectors
> + * bit 9 -- discard sectors
>   *	Informs the lower level device that this range of sectors is no longer
>   *	used by the file system and may thus be freed by the device. Used
>   *	for flash based storage.
> - * bit 7 -- fail fast device errors
> - * bit 8 -- fail fast transport errors
> - * bit 9 -- fail fast driver errors
>   *	Don't want driver retries for any fast fail whatever the reason.
>   * bit 10 -- Tell the IO scheduler not to wait for more requests after this
>  	one has been submitted, even if it is a SYNC request.
>   */
> -#define BIO_RW		0	/* Must match RW in req flags (blkdev.h) */
> -#define BIO_RW_AHEAD	1	/* Must match FAILFAST in req flags */
> -#define BIO_RW_BARRIER	2
> -#define BIO_RW_SYNCIO	3
> -#define BIO_RW_UNPLUG	4
> -#define BIO_RW_META	5
> -#define BIO_RW_DISCARD	6
> -#define BIO_RW_FAILFAST_DEV		7
> -#define BIO_RW_FAILFAST_TRANSPORT	8
> -#define BIO_RW_FAILFAST_DRIVER		9
> -#define BIO_RW_NOIDLE	10
> +enum bio_rw_flags {
> +	BIO_RW,
> +	BIO_RW_FAILFAST_DEV,
> +	BIO_RW_FAILFAST_TRANSPORT,
> +	BIO_RW_FAILFAST_DRIVER,
> +	/* above flags must match REQ_* */
> +	BIO_RW_AHEAD,
> +	BIO_RW_BARRIER,
> +	BIO_RW_SYNCIO,
> +	BIO_RW_UNPLUG,
> +	BIO_RW_META,
> +	BIO_RW_DISCARD,
> +	BIO_RW_NOIDLE,
> +};
>  
>  #define bio_rw_flagged(bio, flag)	((bio)->bi_rw & (1 << (flag)))
>  

I wish there was also an helper to set these bits. it gives me an heart attack
every time I need to:
	bio->bi_rw &= ~(1 << BIO_RW);

> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 49ae079..a0e5ce1 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -98,6 +98,7 @@ enum rq_flag_bits {
>  	__REQ_FAILFAST_DEV,	/* no driver retries of device errors */
>  	__REQ_FAILFAST_TRANSPORT, /* no driver retries of transport errors */
>  	__REQ_FAILFAST_DRIVER,	/* no driver retries of driver errors */
> +	/* above flags must match BIO_RW_* */
>  	__REQ_DISCARD,		/* request to discard sectors */
>  	__REQ_SORTED,		/* elevator knows about this request */
>  	__REQ_SOFTBARRIER,	/* may not be passed by ioscheduler */
> @@ -148,6 +149,9 @@ enum rq_flag_bits {
>  #define REQ_NOIDLE	(1 << __REQ_NOIDLE)
>  #define REQ_IO_STAT	(1 << __REQ_IO_STAT)
>  
> +#define REQ_FAILFAST_MASK	(REQ_FAILFAST_DEV | REQ_FAILFAST_TRANSPORT | \
> +				 REQ_FAILFAST_DRIVER)
> +
>  #define BLK_MAX_CDB	16
>  
>  /*

Thanks
Boaz