Sorry, I'm so rate. I looked into the fix and I think it will work safely on other architectures as well. I think it will also solve the problem with ppc64. I accept and merge this patch.
Thank you for reporting this problem and providing the very difficult fix. Thanks, Masa On 2025/07/10 14:34, Tao Liu wrote: > Kindly ping... > > Sorry to interrupt, could you please merge the patch since there are > few bugs which depend on the backporting of this patch? > > Thanks, > Tao Liu > > > On Fri, Jul 4, 2025 at 7:51 PM Tao Liu <[email protected]> wrote: >> On Fri, Jul 4, 2025 at 6:49 PM HAGIO KAZUHITO(萩尾 一仁) <[email protected]> >> wrote: >>> On 2025/07/04 7:35, Tao Liu wrote: >>>> Hi Petr, >>>> >>>> On Fri, Jul 4, 2025 at 2:31 AM Petr Tesarik <[email protected]> wrote: >>>>> On Tue, 1 Jul 2025 19:59:53 +1200 >>>>> Tao Liu <[email protected]> wrote: >>>>> >>>>>> Hi Kazu, >>>>>> >>>>>> Thanks for your comments! >>>>>> >>>>>> On Tue, Jul 1, 2025 at 7:38 PM HAGIO KAZUHITO(萩尾 一仁) >>>>>> <[email protected]> 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. >>>>> Yes, this is expected. X86 implements a strongly ordered memory model, >>>>> so a "store-to-memory" instruction ensures that the new value is >>>>> immediately observed by other CPUs. >>>>> >>>>> FWIW the current code is wrong even on X86, because it does nothing to >>>>> prevent compiler optimizations. The compiler is then allowed to reorder >>>>> instructions so that the write to page_flag_buf->ready happens after >>>>> other writes; with a bit of bad scheduling luck, the consumer thread >>>>> may see an inconsistent state (e.g. read a stale page_flag_buf->pfn). >>>>> Note that thanks to how compilers are designed (today), this issue is >>>>> more or less hypothetical. Nevertheless, the use of atomics fixes it, >>>>> because they also serve as memory barriers. >>> Thank you Petr, for the information. I was wondering whether atomic >>> operations might be necessary for the other members of page_flag_buf, >>> but it looks like they won't be necessary in this case. >>> >>> Then I was convinced that the issue would be fixed by removing the >>> inconsistency of page_flag_buf->ready. And the patch tested ok, so ack. >>> >> Thank you all for the patch review, patch testing and comments, these >> have been so helpful! >> >> Thanks, >> Tao Liu >> >>> Thanks, >>> Kazu >>> >>>> Thanks a lot for your detailed explanation, it's very helpful! I >>>> haven't thought of the possibility of instruction reordering and >>>> atomic_rw prevents the reorder. >>>> >>>> Thanks, >>>> Tao Liu >>>> >>>>> Petr T >>>>>
