lists.openwall.net   lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  PHC 
Open Source and information security mailing list archives
 
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date:   Sun, 17 Feb 2019 08:44:58 +0000
From:   "Richard W.M. Jones" <rjones@...hat.com>
To:     Pavel Machek <pavel@....cz>
Cc:     kernel list <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...l.org>
Subject: Re: nbd, nbdkit, loopback mounts and memory management

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
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-p2v converts physical machines to virtual machines.  Boot with a
live CD or over the network (PXE) and turn machines into KVM guests.
http://libguestfs.org/virt-v2v

Powered by blists - more mailing lists