linux-kernel - Re: [PATCH] bio: decrease bi_iter.bi

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Date:	Wed, 28 May 2014 17:49:26 +0200
From:	Maurizio Lombardi <mlombard@...hat.com>
To:	Dongsu Park <dongsu.park@...fitbricks.com>
Cc:	linux-kernel@...r.kernel.org,
	Dongsu Park <dongsu.park@...fitbricks.com>,
	Jens Axboe <axboe@...nel.dk>, Jet Chen <jet.chen@...el.com>,
	Maurizio Lombardi <mlombard@...hat.com>, tom.leiming@...il.com
Subject: Re: [PATCH] bio: decrease bi_iter.bi_size by len in the fail path

Hi,

thanks for the patch

On Wed, May 28, 2014 at 05:09:38PM +0200, Dongsu Park wrote:
> From: Dongsu Park <dongsu.park@...fitbricks.com>
> 
> Commit 3979ef4dcf3d1de55a560a3a4016c30a835df44d ("bio-modify-
> __bio_add_page-to-accept-pages-that-dont-start-a-new-segment-v3")
> introduced a regression as reported by Jet Chen.
> That results in a kernel BUG at drivers/block/virtio_blk.c:166.
> 
> To fix that, bi_iter.bi_size must be decreased by len, before
> recounting the number of physical segments.

I don't understand exactly why it must be decreased by len.
Can you provide more info?

Thanks,
Maurizio Lombardi

> 
> Tested on with kernel 3.15.0-rc7-next-20140527 on qemu guest,
> by running xfstests/ext4/271.
> 
> Cc: Jens Axboe <axboe@...nel.dk>
> Cc: Jet Chen <jet.chen@...el.com>
> Cc: Maurizio Lombardi <mlombard@...hat.com>
> Signed-off-by: Dongsu Park <dongsu.park@...fitbricks.com>
> ---
>  block/bio.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/block/bio.c b/block/bio.c
> index 0443694ccbb4..67d7cba1e5fd 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -810,6 +810,7 @@ static int __bio_add_page(struct request_queue *q, struct bio *bio, struct page
>  	bvec->bv_len = 0;
>  	bvec->bv_offset = 0;
>  	bio->bi_vcnt--;
> +	bio->bi_iter.bi_size -= len;
>  	blk_recount_segments(q, bio);
>  	return 0;
>  }
> -- 
> 1.9.1
> 
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