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

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Date:	Wed, 29 Sep 2010 14:21:07 +0900
From:	Jens Axboe <jaxboe@...ionio.com>
To:	Chris Frey <cdfrey@...rsquare.net>
CC:	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>,
	"janjaap@....nl" <janjaap@....nl>,
	"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 2010-09-29 10:29, Chris Frey wrote:
> On Wed, Sep 29, 2010 at 08:10:06AM +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;
>>
>> -- 
> 
> This patch does not fix the corruption issue for me.  I applied the patch
> to 2.6.35.5, and reproduced the "deleted inode referenced" errors
> in both a gentoo and ubuntu guest OS.  It does take longer to reproduce
> though, with this patch.

This seems to imply that the original commit pin pointed is not
the only issue we have in that code atm.

I think we need to find the real fix here, just disabling merging
is not a fix (it's just a nasty work-around for the real bug).

-- 
Jens Axboe

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