linux-kernel - Re: [PATCH] blk-merge: fix blk_recount

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Date:	Tue, 02 Sep 2014 13:01:07 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Ming Lei <ming.lei@...onical.com>, linux-kernel@...r.kernel.org,
	Kick In <pierre-andre.morey@...onical.com>,
	Chris J Arges <chris.j.arges@...onical.com>
Subject: Re: [PATCH] blk-merge: fix blk_recount_segments

Jens Axboe <axboe@...nel.dk> writes:

> On 09/02/2014 09:02 AM, Ming Lei wrote:
>> QUEUE_FLAG_NO_SG_MERGE is set at default for blk-mq devices,
>> so bio->bi_phys_segment computed may be bigger than
>> queue_max_segments(q) for blk-mq devices, then drivers will
>> fail to handle the case, for example, BUG_ON() in
>> virtio_queue_rq() can be triggerd for virtio-blk:
>> 
>> 	https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1359146
>> 
>> This patch fixes the issue by ignoring the QUEUE_FLAG_NO_SG_MERGE
>> flag if the computed bio->bi_phys_segment is bigger than
>> queue_max_segments(q), and the regression is caused by commit
>> 05f1dd53152173(block: add queue flag for disabling SG merging).
>> 
>> Reported-by: Kick In <pierre-andre.morey@...onical.com>
>> Tested-by: Chris J Arges <chris.j.arges@...onical.com>
>> Signed-off-by: Ming Lei <ming.lei@...onical.com>
>
> Thanks Ming, this looks nice and clean. Will apply for 3.17.

This sounds an awful lot like the bug I fixed in dm:

commit 200612ec33e555a356eebc717630b866ae2b694f
Author: Jeff Moyer <jmoyer@...hat.com>
Date:   Fri Aug 8 11:03:41 2014 -0400

    dm table: propagate QUEUE_FLAG_NO_SG_MERGE
    
    Commit 05f1dd5 ("block: add queue flag for disabling SG merging")
    introduced a new queue flag: QUEUE_FLAG_NO_SG_MERGE.  This gets set by
    default in blk_mq_init_queue for mq-enabled devices.  The effect of
    the flag is to bypass the SG segment merging.  Instead, the
    bio->bi_vcnt is used as the number of hardware segments.
    
    With a device mapper target on top of a device with
    QUEUE_FLAG_NO_SG_MERGE set, we can end up sending down more segments
    than a driver is prepared to handle.  I ran into this when backporting
    the virtio_blk mq support.  It triggerred this BUG_ON, in
    virtio_queue_rq:
    
            BUG_ON(req->nr_phys_segments + 2 > vblk->sg_elems);

    ...

Is bcache a stacking driver in this case?  Should it not inherit this
flag, just as DM now does?

-Jeff
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