[dm-devel] Re: dm: bounce_pfn limit added

[Kiyoshi Ueda]

Hi,

On Wed, 31 Oct 2007 08:36:01 +0100, Hannes Reinecke <hare at suse.de> wrote:
> Vasily Averin wrote:
> > Alasdair G Kergon wrote:
> >> So currently we treat bounce_pfn as a property that does not need to be
> >> propagated through the stack.
> >>
> >> But is that the right approach?
> >> - Is there a blk_queue_bounce() missing either from dm or elsewhere?
> >>   (And BTW can the bio_alloc() that lurks within lead to deadlock?)
> >>
> >> Firstly, what's going wrong?
> >> - What is the dm table you are using?  (output of 'dmsetup table')
> >>   - Which dm targets and with how many underlying devices?
> >> - Which underlying driver?
> >> - Is this direct I/O to the block device from userspace, or via some
> >> filesystem or what?
> > 
> > On my testnode I have  6 Gb memory (1Gb normal zone for i386 kernels),
> > i2o hardware and lvm over i2o.
> > 
> > [root at ts10 ~]# dmsetup table
> > vzvg-vz: 0 10289152 linear 80:5 384
> > vzvg-vzt: 0 263127040 linear 80:5 10289536
> > [root at ts10 ~]# cat /proc/partitions
> > major minor  #blocks  name
> > 
> >   80     0  143374336 i2o/hda
> >   80     1     514048 i2o/hda1
> >   80     2    4096575 i2o/hda2
> >   80     3    2040255 i2o/hda3
> >   80     4          1 i2o/hda4
> >   80     5  136721151 i2o/hda5
> >  253     0    5144576 dm-0
> >  253     1  131563520 dm-1
> > 
> > Diotest from LTP test suite with ~1Mb buffer size and files on dm-over-i2o
> > paritions corrupts i2o_iop0_msg_inpool slab.
> > 
> > I2o on this node is able to handle only requests with up to 38 segments. Device
> > mapper correctly creates such requests and as you know it uses
> > max_pfn=BLK_BOUNCE_ANY. When this request translates to underlying device, it
> > clones bio and cleans BIO_SEG_VALID flag.
> > 
> > In this way underlying device calls blk_recalc_rq_segments() to recount number
> > of segments. However blk_recalc_rq_segments uses bounce_pfn=BLK_BOUNCE_HIGH
> > taken from underlying device. As result number of segments become over than
> > max_hw_segments limit.
> > 
> > Unfortunately there is not any checks and when i2o driver handles this incorrect
> > request it fills the memory out of i2o_iop0_msg_inpool slab.
> > 
> We actually had a similar issue with some raid drivers (gdth iirc), and Neil Brown
> did a similar patch for it. These were his comments on it:
> >
> > dm handles max_hw_segments by using an 'io_restrictions' structure
> > that keeps the most restrictive values from all component devices.
> >
> > So it should not allow more than max_hw_segments.
> > 
> > However I just notices that it does not preserve bounce_pfn as a restriction.
> > So when the request gets down to the driver, it may be split up in to more
> > segments than was expected up at the dm level.
> > 
> So I guess we should take this.

How about the case that other dm device is stacked on the dm device?
(e.g. dm-linear over dm-multipath over i2o with bounce_pfn=64GB, and
      the multipath table is changed to i2o with bounce_pfn=1GB.)

With this example, the patch propagates the restriction of i2o
to dm-multipath but not to dm-linear.
So I guess the same problem happens.
Although it may sound like a corner case, such situation could occur
with pvmove of LVM2, for example.
I think we should take care of it too so that system won't be destroyed.
Rejecting to load such table will at least prevent the problem.

Thanks,
Kiyoshi Ueda