linux-kernel - Re: [PATCH] xen/blkfront: remove redundant flush

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Message-ID: <54860EA4.7090705@oracle.com>
Date:	Mon, 08 Dec 2014 15:48:36 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Vitaly Kuznetsov <vkuznets@...hat.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	David Vrabel <david.vrabel@...rix.com>,
	xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
	Andrew Jones <drjones@...hat.com>
Subject: Re: [PATCH] xen/blkfront: remove redundant flush_op

On 12/08/2014 09:17 AM, Vitaly Kuznetsov wrote:
> flush_op is unambiguously defined by feature_flush:
>      REQ_FUA | REQ_FLUSH -> BLKIF_OP_WRITE_BARRIER
>      REQ_FLUSH -> BLKIF_OP_FLUSH_DISKCACHE
>      0 -> 0
> and thus can be removed. This is just a cleanup.
>
> The patch was suggested by Boris Ostrovsky.
>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
> The patch is supposed to be applied after "xen/blkfront: improve protection
> against issuing unsupported REQ_FUA".
> ---
>   drivers/block/xen-blkfront.c | 24 ++++++++++++------------
>   1 file changed, 12 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> index 2e6c103..d1ee233 100644
> --- a/drivers/block/xen-blkfront.c
> +++ b/drivers/block/xen-blkfront.c
> @@ -126,7 +126,6 @@ struct blkfront_info
>   	unsigned int persistent_gnts_c;
>   	unsigned long shadow_free;
>   	unsigned int feature_flush;
> -	unsigned int flush_op;
>   	unsigned int feature_discard:1;
>   	unsigned int feature_secdiscard:1;
>   	unsigned int discard_granularity;
> @@ -479,7 +478,14 @@ static int blkif_queue_request(struct request *req)
>   				 * way.  (It's also a FLUSH+FUA, since it is
>   				 * guaranteed ordered WRT previous writes.)
>   				 */
> -				ring_req->operation = info->flush_op;
> +				if (unlikely(info->feature_flush & REQ_FUA))
> +					ring_req->operation =
> +						BLKIF_OP_WRITE_BARRIER;
> +				else if (likely(info->feature_flush))
> +					ring_req->operation =
> +						BLKIF_OP_FLUSH_DISKCACHE;

To better future-proof it against new flags maybe something like

     switch ( info->feature_flush & (REQ_FLUSH|REQ_FUA) ) {
            case REQ_FLUSH|REQ_FUA: ...
            case REQ_FLUSH: ...
            default: ...

or the if/else equivalent?


> +				else
> +					ring_req->operation = 0;
>   			}
>   			ring_req->u.rw.nr_segments = nseg;
>   		}
> @@ -691,8 +697,8 @@ static void xlvbd_flush(struct blkfront_info *info)
>   	blk_queue_flush(info->rq, info->feature_flush);
>   	printk(KERN_INFO "blkfront: %s: %s: %s %s %s %s %s\n",
>   	       info->gd->disk_name,
> -	       info->flush_op == BLKIF_OP_WRITE_BARRIER ?
> -		"barrier" : (info->flush_op == BLKIF_OP_FLUSH_DISKCACHE ?
> +	       info->feature_flush == (REQ_FLUSH | REQ_FUA) ?
> +		"barrier" : (info->feature_flush == REQ_FLUSH ?

And something similar here?

Thanks.
-boris


>   		"flush diskcache" : "barrier or flush"),
>   	       info->feature_flush ? "enabled;" : "disabled;",
>   	       "persistent grants:",
> @@ -1190,7 +1196,6 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id)
>   				if (error == -EOPNOTSUPP)
>   					error = 0;
>   				info->feature_flush = 0;
> -				info->flush_op = 0;
>   				xlvbd_flush(info);
>   			}
>   			/* fall through */
> @@ -1810,7 +1815,6 @@ static void blkfront_connect(struct blkfront_info *info)
>   		physical_sector_size = sector_size;
>   
>   	info->feature_flush = 0;
> -	info->flush_op = 0;
>   
>   	err = xenbus_gather(XBT_NIL, info->xbdev->otherend,
>   			    "feature-barrier", "%d", &barrier,
> @@ -1823,10 +1827,8 @@ static void blkfront_connect(struct blkfront_info *info)
>   	 *
>   	 * If there are barriers, then we use flush.
>   	 */
> -	if (!err && barrier) {
> +	if (!err && barrier)
>   		info->feature_flush = REQ_FLUSH | REQ_FUA;
> -		info->flush_op = BLKIF_OP_WRITE_BARRIER;
> -	}
>   	/*
>   	 * And if there is "feature-flush-cache" use that above
>   	 * barriers.
> @@ -1835,10 +1837,8 @@ static void blkfront_connect(struct blkfront_info *info)
>   			    "feature-flush-cache", "%d", &flush,
>   			    NULL);
>   
> -	if (!err && flush) {
> +	if (!err && flush)
>   		info->feature_flush = REQ_FLUSH;
> -		info->flush_op = BLKIF_OP_FLUSH_DISKCACHE;
> -	}
>   
>   	err = xenbus_gather(XBT_NIL, info->xbdev->otherend,
>   			    "feature-discard", "%d", &discard,

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