[LU-17116] losetup --direct-io=on trigger LASSERT(ll_dio_aio) in ll_file_io_generic

Related — Thu, 14 Sep 2023 15:40:01 +0000

When trying to use direct I/O on loop devices to format and mount an XFS filesystem backed on a Lustre file, we ran into the following crash:

PID: 51772  TASK: ffff950e22f117c0  CPU: 1   COMMAND: "loop4"
 #0 [ffffafe700c8b8a0] machine_kexec at ffffffffb9659a5e
 #1 [ffffafe700c8b8f8] __crash_kexec at ffffffffb975928d
 #2 [ffffafe700c8b9c0] panic at ffffffffb96b1498
 #3 [ffffafe700c8ba48] ll_direct_IO_impl at ffffffffc1755194 [lustre]
 #4 [ffffafe700c8bb08] generic_file_read_iter at ffffffffb981cd1f
 #5 [ffffafe700c8bb50] vvp_io_read_start at ffffffffc17673be [lustre]
 #6 [ffffafe700c8bbe8] cl_io_start at ffffffffc09e673d [obdclass]
 #7 [ffffafe700c8bc10] cl_io_loop at ffffffffc09e9d1a [obdclass]
 #8 [ffffafe700c8bc48] ll_file_io_generic at ffffffffc170e510 [lustre]
 #9 [ffffafe700c8bd40] ll_file_read_iter at ffffffffc170f9a6 [lustre]
#10 [ffffafe700c8bdb0] lo_rw_aio at ffffffffc08037a9 [loop]
#11 [ffffafe700c8be28] loop_queue_work at ffffffffc0804bc7 [loop]
#12 [ffffafe700c8bee0] kthread_worker_fn at ffffffffb96d5224
#13 [ffffafe700c8bf10] kthread at ffffffffb96d4802
#14 [ffffafe700c8bf50] ret_from_fork at ffffffffba000242

This crash is triggered by `LASSERT(ll_dio_aio)` in `ll_file_io_generic`. The issue is that the loop block device uses `kiocb::ki_flags = IOCB_DIRECT` to trigger a direct I/O but Lustre only looks at `file::f_flags & O_DIRECT` to assess whether we are in DIO or not. This leads to inconsistencies in the expected variables that should be available in the read/write code paths.

This crash was produced with:

truncate -s 100M /mnt/lustre/disk
losetup -f b 4096 -direct-io=on /mnt/lustre/disk

Whamcloud Community JIRA

[LU-17116] losetup --direct-io=on trigger LASSERT(ll_dio_aio) in ll_file_io_generic