Re: [jet.chen@intel.com: [bio] kernel BUG at drivers/block/virtio_blk.c:166!]

[Dongsu Park <dongsu.park@profitbricks.com>]

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Hi,

On 27.05.2014 13:24, 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?

I was also able to reproduce this bug just as reported by Jet Chen,
and now I think I've found out a solution. I'll send out a patch.

Regards,
Dongsu

> 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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