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

[Hannes Reinecke]

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.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
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