[dm-devel] virtio_blk: fix defaults for max_hw_sectors and max_segment_size
snitzer at redhat.com
Wed Nov 26 14:10:59 UTC 2014
On Wed, Nov 26 2014 at 12:58am -0500,
Rusty Russell <rusty at rustcorp.com.au> wrote:
> Mike Snitzer <snitzer at redhat.com> writes:
> > On Thu, Nov 20 2014 at 3:30pm -0500,
> > Michael S. Tsirkin <mst at redhat.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.
Yes, I agree (and understanding the BUG in question will be pursued).
But in the context of the patch I proposed it was irrelevent.
virtio-blk still _should_ fix its limits to reflect those of the block
device it stacks on. My patch was a stop-gap until proper virtio-blk
protocol extensions were added. But you don't seem inclined to care.
> > virtio-blk doesn't get to live in fantasy-land just because it happens
> > to think it is limitless.
> Calm down please.
Sure, but it'd have helped if virtio-blk developers demonstrated
acknowledgement that a stacking block driver should stack the limits of
the underlying device. Instead you decided to trim all related portions
of my reply to mst that were measured and helpful.
> We don't have a sector limit. We have a segment limit, which is set
> above this line.
Then at a minimum max_hw_sectors should reflect that segment limit.
But again, the underlying device has limits that should be stacked up.
Why is that irrelevent to virtio-blk? Plus, this is a matter of not
allowing a user to shoot themselves in the foot by fiddling with
traditional block limits only to find in some kernel (*cough* RHEL6)
they result in BUG.
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