Re: [BUG] JIT symbols not resolved after mprotect() combined vma

[Zhang Boyang <zhangboyang.id@gmail.com>]

Hello,

The problem had been solved. I'm using a old kernel so I encountered 
this problem. I should try my program on latest kernel first.

Here are some pointers about this problem:

https://www.spinics.net/lists/linux-perf-users/msg09833.html
https://lkml.org/lkml/2019/10/31/785

Thank you all!
Zhang Boyang

On 2021/1/9 16:59, Zhang Boyang wrote:
> Hello,
> 
>    I'm using perf to measure my LLVM jitted code performance. However, I 
> found with some condition, perf will unable to resolve or annotate JIT 
> symbols. After some debugging, I found the cause is mprotect() combined 
> two adjacent vma, and report a MMAP2 event of merged vma, confusing perf.
>    I wrote a sample program (sample.c). It creates function computing 
> factorial. First it creates test1() and exec it for 10000 times. Then, 
> it creates test2() and exec both test1() and test2() for 10000 times 
> respectively. As a result, test1() was executed 20000 times, and test2() 
> was executed 10000 times.
>    However, perf report shows:
> 
>    33.33%  a.out    jitted-10742-3.so    [.] test2
>    33.30%  a.out    jitted-10742-2.so    [.] test1
>    33.26%  a.out    [JIT] tid 10742      [.] 0x00007f4b8cb3d010 // test1
> 
>    This is because LLVM create jitted code by using mmap() and 
> mprotect(): (the address is different because of ASLR)
> 
> //// compile test1
> mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) 
> = 0x7fca1a491000
> mprotect(0x7fca1a491000, 4096, PROT_READ|PROT_EXEC) = 0
> //// compile test2
> mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) 
> = 0x7fca1a490000
> mprotect(0x7fca1a490000, 4096, PROT_READ|PROT_EXEC) = 0
> 
>    The second mprotect will merge two adjacent vma, and report a MMAP2 
> event of merged vma. The pref tool saw this:
> 
> 9547033496331 0x2510 [0x60]: PERF_RECORD_MMAP2 10742/10742: 
> [0x7f4b8cb3d000(0x2000) @ 0x7f4b8cb3d000 00:00 0 0]: r-xp //anon
> 9547033524086 0xf00 [0xb0]: PERF_RECORD_MMAP2 10742/10742: 
> [0x7f4b8cb3d000(0x18) @ 0x40 08:02 10224200 1]: --xs 
> /home/zby/src/qemuproject/qemu/.debug/jit/llvm-IR-jit-20210109-ad4573/jitted-10742-2.so 
> 
> 9553628266188 0x103898 [0x60]: PERF_RECORD_MMAP2 10742/10742: 
> [0x7f4b8cb3c000(0x3000) @ 0x7f4b8cb3c000 00:00 0 0]: r-xp //anon
> 9553628283472 0xfb0 [0xb0]: PERF_RECORD_MMAP2 10742/10742: 
> [0x7f4b8cb3c000(0x18) @ 0x40 08:02 10224201 1]: --xs 
> /home/zby/src/qemuproject/qemu/.debug/jit/llvm-IR-jit-20210109-ad4573/jitted-10742-3.so 
> 
> 
>    The event of third line, confusing perf, make it thinks test1() had 
> disappear. Then perf if unable to resolve test1() or annotate it.
> 
> 
> Thank you very much!
> Zhang Boyang