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Date:   Mon, 18 Feb 2019 00:15:14 +0100
From:   Pavel Machek <>
To:     "Richard W.M. Jones" <>,
Cc:     kernel list <>,
        Andrew Morton <>
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:
> 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.)

This looks like quite a good try. I'd try to use mmap() to dirty
memory very quickly.

But Shaun reported it happened somehow often for them, so he might
have a practical test case... better than my theories :-).

Best regards,
(cesky, pictures)

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