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Date: Wed, 28 May 2014 23:42:55 +0800
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Subject: Re: [PATCH] bio: decrease bi_iter.bi_size by len in the fail path
From: Ming Lei <tom.leiming@gmail.com>
To: Dongsu Park <dongsu.park@profitbricks.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Jens Axboe <axboe@kernel.dk>, Jet Chen <jet.chen@intel.com>,
        Maurizio Lombardi <mlombard@redhat.com>
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Hi Dongsu,

On Wed, May 28, 2014 at 11:09 PM, Dongsu Park
<dongsu.park@profitbricks.com> wrote:
> From: Dongsu Park <dongsu.park@profitbricks.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.
>
> Tested on with kernel 3.15.0-rc7-next-20140527 on qemu guest,
> by running xfstests/ext4/271.
>
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: Jet Chen <jet.chen@intel.com>
> Cc: Maurizio Lombardi <mlombard@redhat.com>
> Signed-off-by: Dongsu Park <dongsu.park@profitbricks.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;

Would you mind explaining why bi_iter.bi_size need to be
decreased by 'len'? In the failure path, it wasn't added by
'len', was it?


Thanks,
-- 
Ming Lei