On 1/27/23 03:58, Maurizio Lombardi wrote: > Hello Mike, Dmitry, > > A customer of ours needs an unusual configuration where an iSCSI initiator > runs on the same host of the target; > in other words, the host sees an iSCSI disk which is in fact just a local disk. > > The problem is that under heavy load sometimes the system hangs, > example of backtrace: > > crash> bt 2037117 > PID: 2037117 TASK: ffff8bb4c901dac0 CPU: 0 COMMAND: "iscsi_trx" > #0 [ffffa3f4199db378] __schedule at ffffffff9134b2ed > #1 [ffffa3f4199db408] schedule at ffffffff9134b7c8 > #2 [ffffa3f4199db418] io_schedule at ffffffff9134bbe2 > #3 [ffffa3f4199db428] rq_qos_wait at ffffffff90e61245 > #4 [ffffa3f4199db4b0] wbt_wait at ffffffff90e7bb99 > #5 [ffffa3f4199db4f0] __rq_qos_throttle at ffffffff90e60fc3 > #6 [ffffa3f4199db508] blk_mq_make_request at ffffffff90e5159d > #7 [ffffa3f4199db598] generic_make_request at ffffffff90e4592f > #8 [ffffa3f4199db600] submit_bio at ffffffff90e45bcc > #9 [ffffa3f4199db640] xlog_state_release_iclog at ffffffffc0358cae [xfs] > #10 [ffffa3f4199db668] __xfs_log_force_lsn at ffffffffc0359059 [xfs] > #11 [ffffa3f4199db6d8] xfs_log_force_lsn at ffffffffc035a21f [xfs] > #12 [ffffa3f4199db710] __xfs_iunpin_wait at ffffffffc03454e6 [xfs] > #13 [ffffa3f4199db780] xfs_reclaim_inode at ffffffffc033c203 [xfs] > #14 [ffffa3f4199db7c8] xfs_reclaim_inodes_ag at ffffffffc033c620 [xfs] > #15 [ffffa3f4199db948] xfs_reclaim_inodes_nr at ffffffffc033d851 [xfs] > #16 [ffffa3f4199db960] super_cache_scan at ffffffff90d1cad2 > #17 [ffffa3f4199db9b0] do_shrink_slab at ffffffff90c73e9c > #18 [ffffa3f4199dba20] shrink_slab at ffffffff90c74761 > #19 [ffffa3f4199dbaa0] shrink_node at ffffffff90c7908c > #20 [ffffa3f4199dbb20] do_try_to_free_pages at ffffffff90c79659 > #21 [ffffa3f4199dbb70] try_to_free_pages at ffffffff90c79a5f > #22 [ffffa3f4199dbc10] __alloc_pages_slowpath at ffffffff90cbcd31 > #23 [ffffa3f4199dbd08] __alloc_pages_nodemask at ffffffff90cbd953 > #24 [ffffa3f4199dbd68] sgl_alloc_order at ffffffff90e80e08 > #25 [ffffa3f4199dbdb8] transport_generic_new_cmd at > ffffffffc0972ce5 [target_core_mod] > #26 [ffffa3f4199dbdf8] iscsit_process_scsi_cmd at ffffffffc09eabf5 > [iscsi_target_mod] > #27 [ffffa3f4199dbe18] iscsit_get_rx_pdu at ffffffffc09ec239 > [iscsi_target_mod] > #28 [ffffa3f4199dbed8] iscsi_target_rx_thread at ffffffffc09eda61 > [iscsi_target_mod] > #29 [ffffa3f4199dbf10] kthread at ffffffff90b036c6 > > This is what I think it may happen: > > The rx thread receives an iscsi command, calls sgl_alloc() but the > kernel needs to reclaim memory to satisfy the allocation; the memory > reclaim code starts a flush against the filesystem mounted on top of > the iscsi device, this ends up in a deadlock because the filesystem > needs the > target driver to complete the task, but the iscsi_rx thread is stuck > in sgl_alloc(). > > Sounds correct to you? Yeah, I think nbd and rbd have similar issues. I think they just say don't do that. > > What do you think about using memalloc_noio_*() in the iscsi_rx thread > to prevent the memory reclaim code from starting I/O operations? Any > alternative ideas? I don't think that's the best option because it's a rare use case and it will affect other users. Why can't the user just use tcm loop for the local use case?
