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 21:59:26 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	tj@...nel.org
Cc:	fujita.tomonori@....ntt.co.jp, 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, 01 Apr 2009 21:50:49 +0900
Tejun Heo <tj@...nel.org> wrote:

> FUJITA Tomonori wrote:
> >> * 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.
> 
> Right, ll_back_merge_fn() does that.  Sorry about that.
> 
> >> * 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.
> 
> The only user of blk_rq_append_bio() is scsi_lib.c.  Is Mike
> Christie's code chaining bio's directly into rq?

No, we are not talking about blk_rq_append_bio().

We are talking about the multiple bio handling in blk_rq_map_user,
which is the feature that Mike added long time ago. The feature is
surely necessary for some users. So you can't remote it.
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