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

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Date:	Wed, 01 Apr 2009 21:50:49 +0900
From:	Tejun Heo <tj@...nel.org>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
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()

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?

Thanks.

-- 
tejun
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