The change introduced by the patch is evident in the kernel message log for Thread 0: between page indexes 2 and 1 there's now another function call to write_cache_pages() instead of just another iteration of the for-loop inside one call.
Original kernel: [ 695.276320] Thread 0 running, TID = 3293! [ 695.281210] kp1_pre_handler():070 :: state 1 .... :: pid = 1165, mapping = 0xffff962333263730, comm = 'kworker/u4:2' [ 695.299026] kp1_pre_handler():101 :: state 1 -> 2 :: pid = 1165, mapping = 0xffff962333263730, comm ('kworker/u4:2') is kworker AND wbc->range_cyclic (0x1) is true AND mapping->writeback_index (0x2) is 0x2. [ 695.314808] kp2_pre_handler():119 :: state 2 .... :: pid = 1165, page index = 2 [ 695.322822] kp3_pre_handler():144 :: state 2 .... :: pid = 1165, page index = 2, calling writepage() << ... HERE ... >> [ 695.330308] kp2_pre_handler():119 :: state 2 .... :: pid = 1165, page index = 1 [ 695.334355] kp2_pre_handler():123 :: state 2 -> 3 :: pid = 1165, page index = 1, spin 5 seconds before lock_page()... Test kernel: [ 170.194880] Thread 0 running, TID = 2498! [ 170.200011] kp1_pre_handler():070 :: state 1 .... :: pid = 7, mapping = 0xffff993df9136fb0, comm = 'kworker/u4:0' [ 170.217616] kp1_pre_handler():101 :: state 1 -> 2 :: pid = 7, mapping = 0xffff993df9136fb0, comm ('kworker/u4:0') is kworker AND wbc->range_cyclic (0x1) is true AND mapping->writeback_index (0x2) is 0x2. [ 170.238633] kp2_pre_handler():119 :: state 2 .... :: pid = 7, page index = 2 [ 170.248024] kp3_pre_handler():144 :: state 2 .... :: pid = 7, page index = 2, calling writepage() [ 170.261141] kp1_pre_handler():070 :: state 2 .... :: pid = 7, mapping = 0xffff993df9136fb0, comm = 'kworker/u4:0' [ 170.272150] kp2_pre_handler():119 :: state 2 .... :: pid = 7, page index = 1 [ 170.279860] kp2_pre_handler():123 :: state 2 -> 3 :: pid = 7, page index = 1, spin 5 seconds before lock_page()... ** Changed in: linux (Ubuntu Cosmic) Status: Confirmed => In Progress -- You received this bug notification because you are a member of Kernel Packages, which is subscribed to linux in Ubuntu. https://bugs.launchpad.net/bugs/1824827 Title: tasks doing write()/fsync() hit deadlock in write_cache_pages() Status in linux package in Ubuntu: Invalid Status in linux source package in Cosmic: In Progress Status in linux source package in Disco: Invalid Bug description: [Impact] * Tasks of a multi-threaded workload doing write() and fsync() might deadlock in write_cache_pages(), preventing progress. * The fix addresses a corner case in write_cache_pages() on the range_cyclic implementation which allows the deadlock. * Patch: - commit 64081362e8ff4587b4554087f3cfc73d3e0a4cd7 ("mm/page-writeback.c: fix range_cyclic writeback vs writepages deadlock"), present in v4.20-rc1~92^2~19. [Test Case] * This issue originally has been hit by the 'perforce' (p4d) tool in a XFS filesystem, but it's difficult/rare to occur. * We've written an userspace + kernel module (kprobes-based) to reproduce this problem and verify the test kernel/patch. * The kprobes are strictly tied to particular kernel versions because of the assembly instruction offsets. We'll provide updated versions for -updates and -proposed for verification. * Steps (see output examples in comments): - Userspace part: $ gcc -o test test.c -pthread - Kernel part: $ touch Makefile $ make -C /lib/modules/$(uname -r)/build M=$(pwd) obj-m=kprobe-test.o clean $ make -C /lib/modules/$(uname -r)/build M=$(pwd) obj-m=kprobe-test.o modules - Shorter hung task timeout and higher console logging level to notice the deadlocked tasks sooner, and watch progress: $ echo 10 | sudo tee /proc/sys/kernel/hung_task_timeout_secs $ echo 9 | sudo tee /proc/sys/kernel/printk - Load module / Run userspace part (logging to kernel log) in XFS: $ sudo insmod kprobe-test.ko $ cd /path/to/xfs-mountpoint && sudo sh -c 'stdbuf -oL /path/to/test >/dev/kmsg' $ sudo rmmod kprobe-test You may need to ctrl-z with the original kernel as 'test' doesn't finish. - Check kernel log or watch the system console: $ dmesg Check threads in D state. $ ps -eLo pid,tid,state,comm | grep D | grep -e test -e kworker [Regression Potential] * The patch is small but changes core writeback infrastructure, so there's a chance this may _affect_ some or other behavior that has not been validated with our regression testing; not exactly _break_ it. Please note our regression testing. * This has been verified with 'xfstests' (not only for XFS fs, despite its original name), used by major Linux filesystems for regression testing during development. It's been tested on systems with 24 and 4 CPUs (to exercise differences in scalability, parallelism, and workload) and XFS and ext4 (reporter's environment + Ubuntu's default). No regressions were observed (the set of failed tests is the same in each system and tests failed in the same way). * This has also been verified with 'iozone' for write intensive tests, to exercise the writeback mechanism and no errors were observed. * The reporter has been running the test kernel with the patch for weeks and has not observed any other issues/regressions. [Other Info] * This is only required in Cosmic (for the Bionic HWE kernel), and is already applied in Disco. 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