Re: Hard system lock-ups when using encrypted swap and RAM is exhausted

[Milan Broz <gmazyland@gmail.com>]

Hi,

On 11/12/25 6:18 AM, Aaron Rainbolt wrote:
> Not sure if this is a memory management issue, a LUKS issue, or both,
> so I wrote both mailing lists.

It is not a LUKS issue; cryptsetup/LUKS activates the encrypted device,
so it is only the kernel/dm-crypt handling IOs.

Adding cc to dm-devel as this would be another combination device-mapper
and encrypted swap that could cause issues...

However, could you please specify exactly your storage configuration?

 From the subject, I expected you to have an encrypted swap, but it is
not clear if there are other encrypted devices.

Please paste at least lsblk, lsblk -f output, and also luksDump
(or crypttab if it is not LUKS) for LUKS/dm-crypt configuration.

Thanks,
Milan


> 
> I'm seeing an issue with both the latest mainline kernel (6.18-rc5) and
> Debian 13's 6.12 kernel package. When physical memory fills up, the
> entire system locks up hard, as if it hit rather severe thrashing,
> despite the fact that there appears to be disk cache that can still be
> evicted, and there is ample amounts of swap space remaining (gigabytes
> of it). This issue did not occur with the 6.1 kernel in Debian 12. I'm
> seeing this occur in very low-memory Debian VMs, with between 512 and
> 900 MB RAM, running under VirtualBox and KVM. (I suspect, but have not
> verified, that I'm seeing similar behavior under Xen as well.) These
> VMs generally use a swappiness of 1, though I have seen a lockup occur
> even with a swappiness of 60. The filesystem in use, in case it
> matters, is ext4.
> 
> To reproduce on a system running Linux 6.18-rc5, with :
> 
> * Follow the steps from
>    https://gitlab.com/cryptsetup/cryptsetup/-/wikis/FrequentlyAskedQuestions,
>    section "2.3 How do I set up encrypted swap?", but creating a
>    swapfile rather than a swap partition. I created an 8 GB swapfile
>    with fallocate. Reboot the system when done.
> * In a TTY, open a terminal multiplexer (or something you can abuse as
>    one, Vim works well), and open two terminals. In one terminal, run
>    `htop` so you can observe memory and swap usage.
> * In the `htop` terminal, sort by M_RESIDENT.
> * In the other terminal, create a new file `test.py`, that will
>    gradually fill memory at a relatively fast pace and print an
>    indicator that it's still alive. I used the following code for this:
> 
>      import time
> 
>      count = 0
>      mem_list = []
>      while True:
>          mem_list.append([x for x in range(2048)])
>          count += 1
>          time.sleep(0.002)
>          print(count)
> 
> * Run the script with `python3 test.py`.
> * While the script runs, observe the growing memory usage in `htop`.
>    Swap usage should start at or near 0, RAM usage will gradually
>    increase. Once RAM usage starts getting high, some data will start
>    being swapped out as expected, but after a short while the whole VM
>    will lock up despite there being gigabytes of swap left. (On my KVM
>    VM, the last time htop updated its screen, it showed RAM usage of
>    712M/846M, and swap usage of 328M/7.40G. The python3 process
>    running the script was consuming 551M memory. The VM is entirely
>    unresponsive. Incidentally, the python3 process also was in
>    uninterruptible sleep when htop last updated its screen, but that
>    could mean nothing since it might have come out of sleep between the
>    last screen update and the VM lockup.)
> 
> Under Bookworm with Linux 6.1, the Python script would occasionally
> freeze, but the VM would remain responsive, and the script would
> eventually resume. Even with kernel 6.12, both unencrypted swapfiles and
> swapfiles that are technically unencrypted but live on a LUKS volume
> both behave as expected. It's only swapfiles that are themselves
> encrypted that seem to trigger these lockups.
> 
> I haven't looked at the code at all, but it seems like maybe memory
> LUKS needs available in order to operate is being consumed, thus
> making it impossible to swap anything in and out of the swapfile? That
> seems like it would cause these symptoms or similar, though I don't
> know.
> 
> Let me know if I can provide any further information on the issue. I'm
> happy to bisect the kernel if it will help.
> 
> --
> Aaron