Hi, I notice a regression report on Bugzilla [1]. Quoting from it:
> After upgrading to kernel version 6.4.0 from 6.3.9, I noticed frequent but > random crashes in a user space program. After a lot of reduction, I have > come up with the following reproducer program: > > $ uname -a > Linux jacob 6.4.1-gentoo #1 SMP PREEMPT_DYNAMIC Sat Jul 1 19:02:42 EDT 2023 > x86_64 AMD Ryzen 9 7950X3D 16-Core Processor AuthenticAMD GNU/Linux > $ cat repro.c > #define _GNU_SOURCE > #include <sched.h> > #include <sys/wait.h> > #include <unistd.h> > > void *threadSafeAlloc(size_t n) { > static size_t end_index = 0; > static char buffer[1 << 25]; > size_t start_index = __atomic_load_n(&end_index, __ATOMIC_SEQ_CST); > while (1) { > if (start_index + n > sizeof(buffer)) _exit(1); > if (__atomic_compare_exchange_n(&end_index, &start_index, start_index > + n, 1, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST)) return buffer + start_index; > } > } > > int thread(void *arg) { > size_t i; > size_t n = 1 << 7; > char *items; > (void)arg; > while (1) { > items = threadSafeAlloc(n); > for (i = 0; i != n; i += 1) items[i] = '@'; > for (i = 0; i != n; i += 1) if (items[i] != '@') _exit(2); > } > } > > int main(void) { > static size_t stacks[2][1 << 9]; > size_t i; > for (i = 0; i != 2; i += 1) clone(&thread, &stacks[i] + 1, CLONE_THREAD | > CLONE_VM | CLONE_SIGHAND, NULL); > while (1) { > if (fork() == 0) _exit(0); > (void)wait(NULL); > } > } > $ cc repro.c > $ ./a.out > $ echo $? > 2 > > After tuning the various parameters for my computer, exit code 2, which > indicates that memory corruption was detected, occurs approximately 99% of > the time. Exit code 1, which occurs approximately 1% of the time, means it > ran out of statically-allocated memory before reproducing the issue, and > increasing the memory usage any more only leads to diminishing returns. > There is also something like a 0.1% chance that it segfaults due to memory > corruption elsewhere than in the statically-allocated buffer. > > With this reproducer in hand, I was able to perform the following bisection: > > git bisect start > # status: waiting for both good and bad commits > # bad: [6995e2de6891c724bfeb2db33d7b87775f913ad1] Linux 6.4 > git bisect bad 6995e2de6891c724bfeb2db33d7b87775f913ad1 > # status: waiting for good commit(s), bad commit known > # good: [457391b0380335d5e9a5babdec90ac53928b23b4] Linux 6.3 > git bisect good 457391b0380335d5e9a5babdec90ac53928b23b4 > # good: [d42b1c47570eb2ed818dc3fe94b2678124af109d] Merge tag > 'devicetree-for-6.4-1' of > git://git.kernel.org/pub/scm/linux/kernel/git/robh/linux > git bisect good d42b1c47570eb2ed818dc3fe94b2678124af109d > # bad: [58390c8ce1bddb6c623f62e7ed36383e7fa5c02f] Merge tag > 'iommu-updates-v6.4' of > git://git.kernel.org/pub/scm/linux/kernel/git/joro/iommu > git bisect bad 58390c8ce1bddb6c623f62e7ed36383e7fa5c02f > # good: [888d3c9f7f3ae44101a3fd76528d3dd6f96e9fd0] Merge tag 'sysctl-6.4-rc1' > of git://git.kernel.org/pub/scm/linux/kernel/git/mcgrof/linux > git bisect good 888d3c9f7f3ae44101a3fd76528d3dd6f96e9fd0 > # bad: [86e98ed15b3e34460d1b3095bd119b6fac11841c] Merge tag 'cgroup-for-6.4' > of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup > git bisect bad 86e98ed15b3e34460d1b3095bd119b6fac11841c > # bad: [7fa8a8ee9400fe8ec188426e40e481717bc5e924] Merge tag > 'mm-stable-2023-04-27-15-30' of > git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm > git bisect bad 7fa8a8ee9400fe8ec188426e40e481717bc5e924 > # bad: [0120dd6e4e202e19a0e011e486fb2da40a5ea279] zram: make zram_bio_discard > more self-contained > git bisect bad 0120dd6e4e202e19a0e011e486fb2da40a5ea279 > # good: [fce0b4213edb960859dcc65ea414c8efb11948e1] mm/page_alloc: add helper > for checking if check_pages_enabled > git bisect good fce0b4213edb960859dcc65ea414c8efb11948e1 > # bad: [59f876fb9d68a4d8c20305d7a7a0daf4ee9478a8] mm: avoid passing 0 to > __ffs() > git bisect bad 59f876fb9d68a4d8c20305d7a7a0daf4ee9478a8 > # good: [0050d7f5ee532f92e8ab1efcec6547bfac527973] afs: split > afs_pagecache_valid() out of afs_validate() > git bisect good 0050d7f5ee532f92e8ab1efcec6547bfac527973 > # good: [2ac0af1b66e3b66307f53b1cc446514308ec466d] mm: fall back to mmap_lock > if vma->anon_vma is not yet set > git bisect good 2ac0af1b66e3b66307f53b1cc446514308ec466d > # skip: [0d2ebf9c3f7822e7ba3e4792ea3b6b19aa2da34a] mm/mmap: free > vm_area_struct without call_rcu in exit_mmap > git bisect skip 0d2ebf9c3f7822e7ba3e4792ea3b6b19aa2da34a > # skip: [70d4cbc80c88251de0a5b3e8df3275901f1fa99a] powerc/mm: try VMA > lock-based page fault handling first > git bisect skip 70d4cbc80c88251de0a5b3e8df3275901f1fa99a > # good: [444eeb17437a0ef526c606e9141a415d3b7dfddd] mm: prevent userfaults to > be handled under per-vma lock > git bisect good 444eeb17437a0ef526c606e9141a415d3b7dfddd > # bad: [e06f47a16573decc57498f2d02f9af3bb3e84cf2] s390/mm: try VMA lock-based > page fault handling first > git bisect bad e06f47a16573decc57498f2d02f9af3bb3e84cf2 > # skip: [0bff0aaea03e2a3ed6bfa302155cca8a432a1829] x86/mm: try VMA lock-based > page fault handling first > git bisect skip 0bff0aaea03e2a3ed6bfa302155cca8a432a1829 > # skip: [cd7f176aea5f5929a09a91c661a26912cc995d1b] arm64/mm: try VMA > lock-based page fault handling first > git bisect skip cd7f176aea5f5929a09a91c661a26912cc995d1b > # good: [52f238653e452e0fda61e880f263a173d219acd1] mm: introduce per-VMA lock > statistics > git bisect good 52f238653e452e0fda61e880f263a173d219acd1 > # bad: [c7f8f31c00d187a2c71a241c7f2bd6aa102a4e6f] mm: separate vma->lock from > vm_area_struct > git bisect bad c7f8f31c00d187a2c71a241c7f2bd6aa102a4e6f > # only skipped commits left to test > # possible first bad commit: [c7f8f31c00d187a2c71a241c7f2bd6aa102a4e6f] mm: > separate vma->lock from vm_area_struct > # possible first bad commit: [0d2ebf9c3f7822e7ba3e4792ea3b6b19aa2da34a] > mm/mmap: free vm_area_struct without call_rcu in exit_mmap > # possible first bad commit: [70d4cbc80c88251de0a5b3e8df3275901f1fa99a] > powerc/mm: try VMA lock-based page fault handling first > # possible first bad commit: [cd7f176aea5f5929a09a91c661a26912cc995d1b] > arm64/mm: try VMA lock-based page fault handling first > # possible first bad commit: [0bff0aaea03e2a3ed6bfa302155cca8a432a1829] > x86/mm: try VMA lock-based page fault handling first > > I do not usually see any kernel log output while running the program, just > occasional logs about user space segfaults. See Bugzilla for the full thread. Jacob: Can you repeat bisection please? Why did you skip VMA lock-based page fault commits in your bisection? Anyway, I'm adding it to regzbot: #regzbot introduced: 0bff0aaea03e2a..c7f8f31c00d187 https://bugzilla.kernel.org/show_bug.cgi?id=217624 #regzbot title: Memory corruption in multithreaded user space program while calling fork (possibly caused by trying VMA lock-based page fault) Thanks. [1]: https://bugzilla.kernel.org/show_bug.cgi?id=217624 -- An old man doll... just what I always wanted! - Clara