Hi Kazu, On Wed, Jul 2, 2025 at 12:13 PM HAGIO KAZUHITO(萩尾 一仁) <k-hagio...@nec.com> wrote: > > On 2025/07/01 16:59, Tao Liu wrote: > > Hi Kazu, > > > > Thanks for your comments! > > > > On Tue, Jul 1, 2025 at 7:38 PM HAGIO KAZUHITO(萩尾 一仁) <k-hagio...@nec.com> > > wrote: > >> > >> Hi Tao, > >> > >> thank you for the patch. > >> > >> On 2025/06/25 11:23, Tao Liu wrote: > >>> A vmcore corrupt issue has been noticed in powerpc arch [1]. It can be > >>> reproduced with upstream makedumpfile. > >>> > >>> When analyzing the corrupt vmcore using crash, the following error > >>> message will output: > >>> > >>> crash: compressed kdump: uncompress failed: 0 > >>> crash: read error: kernel virtual address: c0001e2d2fe48000 type: > >>> "hardirq thread_union" > >>> crash: cannot read hardirq_ctx[930] at c0001e2d2fe48000 > >>> crash: compressed kdump: uncompress failed: 0 > >>> > >>> If the vmcore is generated without num-threads option, then no such > >>> errors are noticed. > >>> > >>> With --num-threads=N enabled, there will be N sub-threads created. All > >>> sub-threads are producers which responsible for mm page processing, e.g. > >>> compression. The main thread is the consumer which responsible for > >>> writing the compressed data into file. page_flag_buf->ready is used to > >>> sync main and sub-threads. When a sub-thread finishes page processing, > >>> it will set ready flag to be FLAG_READY. In the meantime, main thread > >>> looply check all threads of the ready flags, and break the loop when > >>> find FLAG_READY. > >> > >> I've tried to reproduce the issue, but I couldn't on x86_64. > > > > Yes, I cannot reproduce it on x86_64 either, but the issue is very > > easily reproduced on ppc64 arch, which is where our QE reported. > > Recently we have enabled --num-threads=N in rhel by default. N == > > nr_cpus in 2nd kernel, so QE noticed the issue. > > I see, thank you for the information. > > > > >> > >> Do you have any possible scenario that breaks a vmcore? I could not > >> think of it only by looking at the code. > > > > I guess the issue only been observed on ppc might be due to ppc's > > memory model, multi-thread scheduling algorithm etc. I'm not an expert > > on those. So I cannot give a clear explanation, sorry... > > ok, I also don't think of how to debug this well.. > > > > > The page_flag_buf->ready is an integer that r/w by main and sub > > threads simultaneously. And the assignment operation, like > > page_flag_buf->ready = 1, might be composed of several assembly > > instructions. Without atomic r/w (memory) protection, there might be > > racing r/w just within the few instructions, which caused the data > > inconsistency. Frankly the ppc assembly consists of more instructions > > than x86_64 for the same c code, which enlarged the possibility of > > data racing. > > > > We can observe the issue without the help of crash, just compare the > > binary output of vmcore generated from the same core file, and > > compress it with or without --num-threads option. Then compare it with > > "cmp vmcore1 vmcore2" cmdline, and cmp will output bytes differ for > > the 2 vmcores, and this is unexpected. > > > >> > >> and this is just out of curiosity, is the issue reproduced with > >> makedumpfile compiled with -O0 too? > > > > Sorry, I haven't done the -O0 experiment, I can do it tomorrow and > > share my findings... > > Thanks, we have to fix this anyway, I want a clue to think about a > possible scenario..
1) Compiled with -O2 flag: [root@ibm-p10-01-lp45 makedumpfile]# ./makedumpfile -d 31 -l ~/vmcore /tmp/out1 Copying data : [100.0 %] / eta: 0s The dumpfile is saved to /tmp/out1. makedumpfile Completed. [root@ibm-p10-01-lp45 makedumpfile]# ./makedumpfile --num-threads=2 -d 31 -l ~/vmcore /tmp/out2 Copying data : [100.0 %] | eta: 0s Copying data : [100.0 %] \ eta: 0s The dumpfile is saved to /tmp/out2. makedumpfile Completed. [root@ibm-p10-01-lp45 makedumpfile]# cd /tmp [root@ibm-p10-01-lp45 tmp]# cmp out1 out2 out1 out2 differ: byte 20786414, line 108064 2) Compiled with -O0 flag: [root@ibm-p10-01-lp45 makedumpfile]# ./makedumpfile -d 31 -l ~/vmcore /tmp/out3 Copying data : [100.0 %] / eta: 0s The dumpfile is saved to /tmp/out3. makedumpfile Completed. [root@ibm-p10-01-lp45 makedumpfile]# ./makedumpfile --num-threads=2 -d 31 -l ~/vmcore /tmp/out4 Copying data : [100.0 %] | eta: 0s Copying data : [100.0 %] \ eta: 0s The dumpfile is saved to /tmp/out4. makedumpfile Completed. [root@ibm-p10-01-lp45 makedumpfile]# cd /tmp [root@ibm-p10-01-lp45 tmp]# cmp out3 out4 out3 out4 differ: byte 23948282, line 151739 Looks to me the O0/O2 have no difference for this case. If no problem, the /tmp/outX generated from both single/multi thread should be exactly the same, however the cmp reports there are differences. With the v2 patch applied, there is no such difference: [root@ibm-p10-01-lp45 makedumpfile]# ./makedumpfile -d 31 -l ~/vmcore /tmp/out5 Copying data : [100.0 %] / eta: 0s The dumpfile is saved to /tmp/out5. makedumpfile Completed. [root@ibm-p10-01-lp45 makedumpfile]# ./makedumpfile --num-threads=2 -d 31 -l ~/vmcore /tmp/out6 Copying data : [100.0 %] | eta: 0s Copying data : [100.0 %] \ eta: 0s The dumpfile is saved to /tmp/out6. makedumpfile Completed. [root@ibm-p10-01-lp45 makedumpfile]# cmp /tmp/out5 /tmp/out6 [root@ibm-p10-01-lp45 makedumpfile]# Thanks, Tao Liu > > Thanks, > Kazu
