linux-kernel - Re: [jet.chen@...el.com: [bio] kernel BUG at drivers/block/virtio

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Date:	Wed, 28 May 2014 23:16:17 +0800
From:	Jet Chen <jet.chen@...el.com>
To:	Maurizio Lombardi <mlombard@...hat.com>,
	Ming Lei <tom.leiming@...il.com>
CC:	Jens Axboe <axboe@...nel.dk>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	LKML <linux-kernel@...r.kernel.org>, lkp@...org,
	Fengguang Wu <fengguang.wu@...el.com>
Subject: Re: [jet.chen@...el.com: [bio] kernel BUG at drivers/block/virtio_blk.c:166!]

On 05/27/2014 07:24 PM, Maurizio Lombardi wrote:
> On Tue, May 27, 2014 at 10:43:59AM +0200, Maurizio Lombardi wrote:
>>
>> But now I'm suspicious of this part of commit 3979ef4dcf:
>>
>>  failed:
>>         bvec->bv_page = NULL;
>>         bvec->bv_len = 0;
>>         bvec->bv_offset = 0;
>>         bio->bi_vcnt--;  <----------------
>>         blk_recount_segments(q, bio);
>>         return 0;
>>
>> Is decreasing bi_vcnt sufficient to guarantee that blk_recount_segments()
>> recalculates the correct number of physical segments?
>> Looking at the __blk_recalc_rq_segments() it appears it may not be the case.
>>
>> The question is how can we restore the correct number of physical segments in case
>> of failure without breaking anything...
>>
> 
> If my hypothesis is correct, the following patch should trigger a kernel panic,
> Jet Chen, can you try it and let me know whether the BUG_ON is hit or not?

Sorry for late respond. Dongsu has sent a patch for this issue.
	message-id <1401289778-9840-1-git-send-email-dongsu.park@...fitbricks.com>
Do you still need me to test the following patch ?

> 
> diff --git a/block/bio.c b/block/bio.c
> index 0443694..763868f 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -701,6 +701,7 @@ static int __bio_add_page(struct request_queue *q, struct bio *bio, struct page
>  			  unsigned int max_sectors)
>  {
>  	int retried_segments = 0;
> +	unsigned int phys_segments_orig;
>  	struct bio_vec *bvec;
>  
>  	/*
> @@ -751,6 +752,9 @@ static int __bio_add_page(struct request_queue *q, struct bio *bio, struct page
>  	if (bio->bi_vcnt >= bio->bi_max_vecs)
>  		return 0;
>  
> +	blk_recount_segments(q, bio);
> +	phys_segments_orig = bio->bi_phys_segments;
> +
>  	/*
>  	 * setup the new entry, we might clear it again later if we
>  	 * cannot add the page
> @@ -811,6 +815,7 @@ static int __bio_add_page(struct request_queue *q, struct bio *bio, struct page
>  	bvec->bv_offset = 0;
>  	bio->bi_vcnt--;
>  	blk_recount_segments(q, bio);
> +	BUG_ON(phys_segments_orig != bio->bi_phys_segments);
>  	return 0;
>  }
> 
> 
> Regards,
> Maurizio Lombardi 
> 
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