linux-kernel - Re: [PATCH v1 12/54] dm: limit the max bio size as BIO_MAX_PAGES * PAGE

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Date:   Tue, 3 Jan 2017 11:43:13 -0500
From:   Mike Snitzer <snitzer@...hat.com>
To:     Ming Lei <tom.leiming@...il.com>
Cc:     Jens Axboe <axboe@...com>, linux-kernel@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>,
        linux-block@...r.kernel.org,
        "maintainer:DEVICE-MAPPER LVM, Shaohua Li <shli@...nel.org>,
        linux-raid@...r.kernel.org open list:SOFTWARE RAID Multiple DisksSUPPORT" 
        <dm-devel@...hat.com>, Alasdair Kergon <agk@...hat.com>
Subject: Re: [PATCH v1 12/54] dm: limit the max bio size as BIO_MAX_PAGES *
 PAGE_SIZE

On Tue, Dec 27 2016 at 10:56am -0500,
Ming Lei <tom.leiming@...il.com> wrote:

> For BIO based DM, some targets aren't ready for dealing with
> bigger incoming bio than 1Mbyte, such as crypt target.
> 
> Signed-off-by: Ming Lei <tom.leiming@...il.com>
> ---
>  drivers/md/dm.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> index 3086da5664f3..6139bf7623f7 100644
> --- a/drivers/md/dm.c
> +++ b/drivers/md/dm.c
> @@ -899,7 +899,16 @@ int dm_set_target_max_io_len(struct dm_target *ti, sector_t len)
>  		return -EINVAL;
>  	}
>  
> -	ti->max_io_len = (uint32_t) len;
> +	/*
> +	 * BIO based queue uses its own splitting. When multipage bvecs
> +	 * is switched on, size of the incoming bio may be too big to
> +	 * be handled in some targets, such as crypt.
> +	 *
> +	 * When these targets are ready for the big bio, we can remove
> +	 * the limit.
> +	 */
> +	ti->max_io_len = min_t(uint32_t, len,
> +			       (BIO_MAX_PAGES * PAGE_SIZE));
>  
>  	return 0;
>  }
> -- 
> 2.7.4

dm_set_target_max_io_len() is already meant to be called by the .ctr
hook for each DM target.  So why not just have the dm-crypt target (and
other targets if needed) pass your reduced $len?

That way only targets that need to be fixed (e.g. dm-crypt) impose this
limit.