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.
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
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