linux-kernel - Re: [PATCH 8/8] blk-map: reimplement blk_rq_map_user() using blk_rq_map_user

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Date:	Wed, 1 Apr 2009 21:44:09 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	tj@...nel.org
Cc:	axboe@...nel.dk, bharrosh@...asas.com,
	linux-kernel@...r.kernel.org, tj@...el.org
Subject: Re: [PATCH 8/8] blk-map: reimplement blk_rq_map_user() using
 blk_rq_map_user_iov()

On Wed,  1 Apr 2009 20:04:44 +0900
Tejun Heo <tj@...nel.org> wrote:

> Impact: subtle bugs fixed, cleanup
> 
> blk_rq_map_user() supported multi-bio mapping by calling
> bio_map/copy_user() multiple times and linking the resulting bios into
> rq; however, this had subtle bugs.
> 
> * Because each call to bio_map/copy_user() is independent, segment
>   limit check was done only per each bio, so it was possible to create
>   requests which are larger than the driver and hardware limits, which
>   could lead to disastrous outcome.

What do you mean? blk_rq_append_bio properly checks the segment and
limit, I think.


> * Layers under FS may call blk_rq_map*() functions during request
>   processing.  Under severe memory pressure and with enough bad luck,
>   this can lead to deadlock.  As fs bvec pool is quite small, the
>   possibility isn't completely theoretical.
>
> This patch reimplement blk_rq_map_user() in terms of
> blk_rq_map_user_iov() which doesn't support multi-bio mappping and
> drop multi bio handling from blk_rq_unmap_user().  Note that with the
> previous patch to remove bio max size limit and to add null mapping
> support to blk_rq_map_user_iov(), this change doesn't remove any
> functionality.

I don't think that we can drop multi bio handling from
blk_rq_unmap_user(). It may make some users angry. Mike Christie added
it because it was necessary.
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