linux-kernel - Re: [PATCH] xen/blkfront: improve protection against issuing unsupported REQ

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Date:	Mon, 03 Nov 2014 13:22:00 +0100
From:	Laszlo Ersek <lersek@...hat.com>
To:	Vitaly Kuznetsov <vkuznets@...hat.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>,
	xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
	Jeff Moyer <jmoyer@...hat.com>,
	Andrew Jones <drjones@...hat.com>
Subject: Re: [PATCH] xen/blkfront: improve protection against issuing unsupported
 REQ_FUA

On 10/27/14 14:44, Vitaly Kuznetsov wrote:
> Guard against issuing unsupported REQ_FUA and REQ_FLUSH was introduced
> in d11e61583 and was factored out into blkif_request_flush_valid() in
> 0f1ca65ee. However:
> 1) This check in incomplete. In case we negotiated to feature_flush = REQ_FLUSH
>    and flush_op = BLKIF_OP_FLUSH_DISKCACHE (so FUA is unsupported) FUA request
>    will still pass the check.
> 2) blkif_request_flush_valid() is misnamed. It is bool but returns true when
>    the request is invalid.
> 3) When blkif_request_flush_valid() fails -EIO is being returned. It seems that
>    -EOPNOTSUPP is more appropriate here.
> Fix all of the above issues.
> 
> This patch is based on the original patch by Laszlo Ersek and a comment by
> Jeff Moyer.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  drivers/block/xen-blkfront.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> index 5ac312f..2e6c103 100644
> --- a/drivers/block/xen-blkfront.c
> +++ b/drivers/block/xen-blkfront.c
> @@ -582,12 +582,14 @@ static inline void flush_requests(struct blkfront_info *info)
>  		notify_remote_via_irq(info->irq);
>  }
>  
> -static inline bool blkif_request_flush_valid(struct request *req,
> -					     struct blkfront_info *info)
> +static inline bool blkif_request_flush_invalid(struct request *req,
> +					       struct blkfront_info *info)
>  {
>  	return ((req->cmd_type != REQ_TYPE_FS) ||
> -		((req->cmd_flags & (REQ_FLUSH | REQ_FUA)) &&
> -		!info->flush_op));
> +		((req->cmd_flags & REQ_FLUSH) &&
> +		 !(info->feature_flush & REQ_FLUSH)) ||
> +		((req->cmd_flags & REQ_FUA) &&
> +		 !(info->feature_flush & REQ_FUA)));
>  }
>  
>  /*
> @@ -612,8 +614,8 @@ static void do_blkif_request(struct request_queue *rq)
>  
>  		blk_start_request(req);
>  
> -		if (blkif_request_flush_valid(req, info)) {
> -			__blk_end_request_all(req, -EIO);
> +		if (blkif_request_flush_invalid(req, info)) {
> +			__blk_end_request_all(req, -EOPNOTSUPP);
>  			continue;
>  		}
>  
> 

Not sure if there has been some feedback yet (I can't see anything
threaded with this message in my inbox).

FWIW I consulted "Documentation/block/writeback_cache_control.txt" for
this review. Apparently, REQ_FLUSH forces out "previously completed
write requests", whereas REQ_FUA delays the IO completion signal for
*this* request until "the data has been committed to non-volatile
storage". So, indeed, support for REQ_FLUSH only does not guarantee that
REQ_FUA can be served.

Reviewed-by: Laszlo Ersek <lersek@...hat.com>

Thanks
Laszlo
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