linux-kernel - Re: [PATCH 1/1] um: ubd: Fix data corruption

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Message-id: <1285721308.9775.3.camel@voyager>
Date:	Wed, 29 Sep 2010 02:48:28 +0200
From:	Janjaap Bos <janjaap@....nl>
To:	Jens Axboe <jaxboe@...ionio.com>
Cc:	Chris Frey <cdfrey@...rsquare.net>,
	Richard Weinberger <richard@....at>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"jdike@...toit.com" <jdike@...toit.com>,
	"user-mode-linux-devel@...ts.sourceforge.net" 
	<user-mode-linux-devel@...ts.sourceforge.net>,
	"user-mode-linux-user@...ts.sourceforge.net" 
	<user-mode-linux-user@...ts.sourceforge.net>,
	"geert@...ux-m68k.org" <geert@...ux-m68k.org>,
	"martin.petersen@...cle.com" <martin.petersen@...cle.com>,
	"adobriyan@...il.com" <adobriyan@...il.com>,
	"syzop@...nscan.org" <syzop@...nscan.org>
Subject: Re: [PATCH 1/1] um: ubd: Fix data corruption

On Wed, 2010-09-29 at 08:10 +0900, Jens Axboe wrote:

> It looks like that if we need to restart the requeue, then
> we use the initial position and not the current index. Does
> this help?
> 
> diff --git a/arch/um/drivers/ubd_kern.c b/arch/um/drivers/ubd_kern.c
> index 1bcd208..81ee063 100644
> --- a/arch/um/drivers/ubd_kern.c
> +++ b/arch/um/drivers/ubd_kern.c
> @@ -162,7 +162,7 @@ struct ubd {
>  	spinlock_t lock;
>  	struct scatterlist sg[MAX_SG];
>  	struct request *request;
> -	int start_sg, end_sg;
> +	int start_sg, end_sg, rq_off;
>  };
>  
>  #define DEFAULT_COW { \
> @@ -187,6 +187,7 @@ struct ubd {
>  	.request =		NULL, \
>  	.start_sg =		0, \
>  	.end_sg =		0, \
> +	.rq_off =		0, \
>  }
>  
>  /* Protected by ubd_lock */
> @@ -1241,10 +1242,11 @@ static void do_ubd_request(struct request_queue *q)
>  			dev->request = req;
>  			dev->start_sg = 0;
>  			dev->end_sg = blk_rq_map_sg(q, req, dev->sg);
> +			dev->rq_off = 0;
>  		}
>  
>  		req = dev->request;
> -		sector = blk_rq_pos(req);
> +		sector = blk_rq_pos(req) + dev->rq_off;
>  		while(dev->start_sg < dev->end_sg){
>  			struct scatterlist *sg = &dev->sg[dev->start_sg];
>  
> @@ -1273,6 +1275,7 @@ static void do_ubd_request(struct request_queue *q)
>  			}
>  
>  			dev->start_sg++;
> +			dev->rq_off += sg->length >> 9;
>  		}
>  		dev->end_sg = 0;
>  		dev->request = NULL;
> 

Yes, this works. I quickly tested it on current git version, and it does
not require the work-around. So far no corruption seen.

Thanks!
-Janjaap


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