Hi Tao, On 2025/07/02 13:36, Tao Liu wrote: > 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]#
thank you for testing! sorry one more thing, does --num-threads=1 break the vmcore? Thanks, Kazu
