On Wed, Mar 9, 2016 at 11:15 PM, John David Anglin <dave.anglin@xxxxxxxx> wrote:
> On 2016-03-09 9:43 AM, Ming Lei wrote:
>>>
>>> We've provided all the information you asked for, what's the next step
>>> >on this, or do we have to unwind the bio splitting code with reverts
>>> >until it starts working?
>>
>> John, Helge, and I did discuss the problem for a while privately, and
>> looks
>> it is related with compiler. Last time, I sent one patch which can make
>> the
>> issue disappear, but the main change is just invovled with the below:
>>
>> struct bio_vec {
>> struct page *bv_page;
>> - unsigned int bv_len;
>> + unsigned int bv_seg:8;
>> + unsigned int bv_len:24;
>> unsigned int bv_offset;
>> };
>>
>> Maybe John and Helge have some update recently?
>>
>> The logic in blk_bio_segment_split() is correct, and it does respect the
>> max
>> segment size limit.
>
> Helge has found that tagging blk_bio_segment_split() with "__attribute__
> ((optimize("O0")))"
> makes the issue disappear. The bug remains if one just adds bv_len to the
> struct without the
> bit fields. Maybe problem is evident from following output which I sent to
> Ming and Helge
> last weekend?
>
> blk_rq_map_sg: merge bug: 3 2, extra_len 0, dma_drain 0
> check_bvec: dump bvec for 000000007e4efdc0(f:24490000, t:1)
> 0: 0 4096 246503 000000007e4a4f00(0, 94208, 1)
> 1: 0 4096 246504 000000007e4a4f00(0, 94208, 1)
> 2: 0 4096 246505 000000007e4a4f00(0, 94208, 1)
> 3: 0 4096 246506 000000007e4a4f00(0, 94208, 1)
The above 4 io vectors belong to one same segment since
they are contineous physically from the 3rd column of PFN,
but the vector 4(the below one) isn't contineous with above 3
vectors, so the following one starts the 2nd segment.
> 4: 0 4096 246538 000000007e4a4f00(0, 94208, 2)
> 5: 0 4096 246539 000000007e4a4f00(0, 94208, 2)
> 6: 0 4096 246540 000000007e4a4f00(0, 94208, 2)
> 7: 0 4096 246541 000000007e4a4f00(0, 94208, 2)
> 8: 0 4096 246542 000000007e4a4f00(0, 94208, 2)
> 9: 0 4096 246543 000000007e4a4f00(0, 94208, 2)
> 10: 0 4096 246544 000000007e4a4f00(0, 94208, 2)
> 11: 0 4096 246545 000000007e4a4f00(0, 94208, 2)
> 12: 0 4096 246546 000000007e4a4f00(0, 94208, 2)
> 13: 0 4096 246547 000000007e4a4f00(0, 94208, 2)
> 14: 0 4096 246548 000000007e4a4f00(0, 94208, 2)
> 15: 0 4096 246549 000000007e4a4f00(0, 94208, 2)
> 16: 0 4096 246550 000000007e4a4f00(0, 94208, 2)
> 17: 0 4096 246551 000000007e4a4f00(0, 94208, 2)
> 18: 0 4096 246552 000000007e4a4f00(0, 94208, 2)
> 19: 0 4096 246553 000000007e4a4f00(0, 94208, 2)
The above 16 vectors are contineous physically, but the segment
size becomes 64K now, so blk_bio_segment_split() should have
seen that and started to split the bio.
> 20: 0 4096 246554 000000007e4a4f00(0, 94208, 2)
> 21: 0 4096 246555 000000007e4a4f00(0, 94208, 2)
> 22: 0 4096 246556 000000007e4a4f00(0, 94208, 2)
Unfortunately the bio isn't splitted at all, that means the following
code is run incorrectly:
if (seg_size + bv.bv_len > queue_max_segment_size(q))
goto new_segment;
seg_size should be 64K, and bv.bv_len should be 4K, so
the sum between the two should be bigger than
64K(queue_max_segment_size(q)). Unfortunately, the
code is optimized as run incorrectly.
> Kernel panic - not syncing: bad block merge
>
> It seems segment 1 is too small and segment 2 too big?
segment 1 is correct, and segment 2 becomes too big, but
__blk_segment_map_sg() runs correctly and figured out
this bio has 3 segments, so causes the oops.
>
> The general plan is to disable inlining (maybe move blk_bio_segment_split()
> to a separate
> function) to try to figure out what is miscompiled.
>
> As you say, this is probably a GCC bug. However, it's likely a middle-end
> or optimization
> bug in the common GCC code.
>
> Dave
>
>
> --
> John David Anglin dave.anglin@xxxxxxxx
>
--
Ming Lei
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