Re: [BUG] "block: make generic_make_request handle arbitrarily sized bios" breaks boot on parisc-linux

[Helge Deller <deller@gmx.de>]

On 09.03.2016 16:15, John David Anglin 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)
>             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)
>            20: 0 4096 246554 000000007e4a4f00(0, 94208, 2)
>            21: 0 4096 246555 000000007e4a4f00(0, 94208, 2)
>            22: 0 4096 246556 000000007e4a4f00(0, 94208, 2)
> Kernel panic - not syncing: bad block merge
> 
> It seems segment 1 is too small and segment 2 too big?
> 
> 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.

Right.
I just succeeded in reproducing the bug with moving blk_bio_segment_split() into an own file
(and with "extern" instead of "static" in blk-merge.c). When compiled with -O2 it still crashes.
So, next step is to analyze what gcc does wrong when compiling this function.
It should get easier now to find the reason, since we have a smaller reproducer now.

Helge
 
> 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
>