Re: nbd, nbdkit, loopback mounts and memory management

["Richard W.M. Jones" <rjones@redhat.com>]

So not to dispute that this could be a bug, but I couldn't cause a
deadlock.  I wonder if you can see something wrong with my method?

*** Set up ***

- kernel 5.0.0-0.rc3.git0.1.fc30.x86_64
- nbd-client 3.19-1.fc30
- nbdkit 1.11.5 (git commit ef9d1978ce28)

Baremetal machine was booted with mem=2G to artificially limit the
RAM.  The machine has 64G of swap.

# free -m
              total        used        free      shared  buff/cache   available
Mem:           1806         329        1383           0          93        1357
Swap:         65535         179       65356

*** Method ***

I started nbdkit as a 4G RAM disk:

  ./nbdkit memory size=4G

This is implemented as a sparse array with a 2 level page table, and
should allocate (using malloc) every time a new area of the disk is
written to.  Exact implementation is here:
https://github.com/libguestfs/nbdkit/tree/master/common/sparse

I started nbd-client using the -swap option which uses
mlockall(MCL_FUTURE) to lock the client into RAM.

  nbd-client -b 512 -swap localhost /dev/nbd0

I then created a filesystem on the RAM disk, mounted it, and copied a
3G file into it.  I tried this various ways, but the variation I was
eventually happy with was:

  mke2fs /dev/nbd0
  mount /dev/nbd0 /tmp/mnt

  dd if=/dev/zero of=/tmp/big bs=1M count=3000
  cp /tmp/big /tmp/mnt/big

I couldn't get any kind of deadlock or failure in this test.

(Note that if you repeat the test several times, in theory you could
delete the file and fstrim the filesystem, but when I was testing it
to be sure I unmounted everything and killed and restarted nbdkit
between each test.)

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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