On Mon, Feb 15, 2016 at 12:29 AM, Sagi Grimberg
[off-list ref] wrote:

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Hi Sagi,

Hey,

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But I don't think simply not cloning the biovecs is the right thing
to do in the end.  This must be something with the bvec iterators.

I agree with Christoph, and there might be issues somewhere.

Me too, it was just an isolation step...

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 From the log:
iser: sg[0] dma_addr:0x85FC06000 off:0x0 sz:0x200 dma_len:0x200
iser: sg[1] dma_addr:0x860334000 off:0x0 sz:0x200 dma_len:0x200 <-- gap
iser: sg[2] dma_addr:0x860335000 off:0x0 sz:0x200 dma_len:0x200 <-- gap

The above gap shouldn't have come since blk_bio_segment_split() splits
out one new bio if gap is detected.

Sort of the following code can be added in driver or prep_fn to check if
bvec of  the rq is correct:

rq_for_each_segment(bvec, sc->request, iter) {
      //check if there is gap between bvec
}

I added this indication and the gap detection does trigger a bio
split.

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I don't know how to use iser, and looks everything works fine after
I setup virt boundary as 4095 for null_blk by the attachment
patch.

That's probably because it's artificial and there is no HW with a real
limitation...

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Full quote for Ming:

On Sun, Feb 14, 2016 at 04:02:18PM +0200, Sagi Grimberg wrote:

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I'm bisecting now, there are a couple of patches from Ming in
the area of the bio splitting code...

CC'ing Ming, Linux-block and Linux-nvme as iser is identical to nvme
wrt the virtual boundary so I think nvme will break as well.

The bisected commit is merged to v4.3, and looks no such kind of
report from nvme.

I'm wandering how can that be... because clearly iser is seeing gaps
which like nvme, it can't handle those. Maybe this is scsi specific?

I can reproduce the issue now, and it is easy to trigger it via your test
code on scsi device, but a bit difficult to get it on null_blk.

Turns out it is a block core issue, and it is in bio_will_gap() which gets
the last bvec via 'bi_io_vec[prev->bi_vcnt - 1]' directly.  I have posted
out one patchset for fixing the issue:

http://marc.info/?l=linux-kernel&m=145551975429092&w=2

Thanks,
Ming
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