linux-kernel - Re: virtio_blk: fix defaults for max_hw_sectors and max_segment

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Date:	Wed, 26 Nov 2014 16:28:27 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Mike Snitzer <snitzer@...hat.com>,
	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	axboe@...nel.dk, linux-kernel@...r.kernel.org,
	martin.petersen@...cle.com, hch@...radead.org, dm-devel@...hat.com
Subject: Re: virtio_blk: fix defaults for max_hw_sectors and max_segment_size

Mike Snitzer <snitzer@...hat.com> writes:
> On Thu, Nov 20 2014 at  3:30pm -0500,
> Michael S. Tsirkin <mst@...hat.com> wrote:
>
>> On Thu, Nov 20, 2014 at 02:00:59PM -0500, Mike Snitzer wrote:
>> > virtio_blk incorrectly established -1U as the default for these
>> > queue_limits.  Set these limits to sane default values to avoid crashing
>> > the kernel.
> ...
>> > Attempting to mkfs.xfs against a thin device from this thin-pool quickly
>> > resulted in fs/direct-io.c:dio_send_cur_page()'s BUG_ON.
>> 
>> Why exactly does it BUG_ON?
>> Did some memory allocation fail?
>
> No idea, kernel log doesn't say.. all it has is "kernel BUG" pointing to
> fs/direct-io.c:dio_send_cur_page()'s BUG_ON.
>
> I could dig deeper on _why_ but honestly, there really isn't much point.

There is *always* a point in understanding the code you are modifying.

> virtio-blk doesn't get to live in fantasy-land just because it happens
> to think it is limitless.

Calm down please.

We don't have a sector limit.  We have a segment limit, which is set
above this line.

Cheers,
Rusty.
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