v5.10.1 xfs deadlock

[Donald Buczek]

Dear xfs developer,

I was doing some testing on a Linux 5.10.1 system with two 100 TB xfs filesystems on md raid6 raids.

The stress test was essentially `cp -a`ing a Linux source repository with two threads in parallel on each filesystem.

After about on hour, the processes to one filesystem (md1) blocked, 30 minutes later the process to the other filesystem (md0) did.

    root      7322  2167  0 Dec16 pts/1    00:00:06 cp -a /jbod/M8068/scratch/linux /jbod/M8068/scratch/1/linux.018.TMP
    root      7329  2169  0 Dec16 pts/1    00:00:05 cp -a /jbod/M8068/scratch/linux /jbod/M8068/scratch/2/linux.019.TMP
    root     13856  2170  0 Dec16 pts/1    00:00:08 cp -a /jbod/M8067/scratch/linux /jbod/M8067/scratch/2/linux.028.TMP
    root     13899  2168  0 Dec16 pts/1    00:00:05 cp -a /jbod/M8067/scratch/linux /jbod/M8067/scratch/1/linux.027.TMP

Some info from the system (all stack traces, slabinfo) is available here: https://owww.molgen.mpg.de/~buczek/2020-12-16.info.txt

It stands out, that there are many (549 for md0, but only 10 for md1)  "xfs-conv" threads all with stacks like this

    [<0>] xfs_log_commit_cil+0x6cc/0x7c0
    [<0>] __xfs_trans_commit+0xab/0x320
    [<0>] xfs_iomap_write_unwritten+0xcb/0x2e0
    [<0>] xfs_end_ioend+0xc6/0x110
    [<0>] xfs_end_io+0xad/0xe0
    [<0>] process_one_work+0x1dd/0x3e0
    [<0>] worker_thread+0x2d/0x3b0
    [<0>] kthread+0x118/0x130
    [<0>] ret_from_fork+0x22/0x30

xfs_log_commit_cil+0x6cc is

  xfs_log_commit_cil()
    xlog_cil_push_background(log)
      xlog_wait(&cil->xc_push_wait, &cil->xc_push_lock);

Some other threads, including the four "cp" commands are also blocking at xfs_log_commit_cil+0x6cc

There are also single "flush" process for each md device with this stack signature:

    [<0>] xfs_map_blocks+0xbf/0x400
    [<0>] iomap_do_writepage+0x15e/0x880
    [<0>] write_cache_pages+0x175/0x3f0
    [<0>] iomap_writepages+0x1c/0x40
    [<0>] xfs_vm_writepages+0x59/0x80
    [<0>] do_writepages+0x4b/0xe0
    [<0>] __writeback_single_inode+0x42/0x300
    [<0>] writeback_sb_inodes+0x198/0x3f0
    [<0>] __writeback_inodes_wb+0x5e/0xc0
    [<0>] wb_writeback+0x246/0x2d0
    [<0>] wb_workfn+0x26e/0x490
    [<0>] process_one_work+0x1dd/0x3e0
    [<0>] worker_thread+0x2d/0x3b0
    [<0>] kthread+0x118/0x130
    [<0>] ret_from_fork+0x22/0x30

xfs_map_blocks+0xbf is the

    xfs_ilock(ip, XFS_ILOCK_SHARED);

in xfs_map_blocks().

The system is low on free memory

    MemTotal:       197587764 kB
    MemFree:          2196496 kB
    MemAvailable:   189895408 kB

but responsive.

I have an out of tree driver for the HBA ( smartpqi 2.1.6-005 pulled from linux-scsi) , but it is unlikely that this blocking is related to that, because the md block devices itself are responsive (`xxd /dev/md0` )

I can keep the system in the state for a while. Is there an idea what was going from or an idea what data I could collect from the running system to help? I have full debug info and could walk lists or retrieve data structures with gdb.

Best
  Donald