Re: [dm-crypt] LUKS2 SSD swap and system freeze

[Milan Broz <gmazyland@gmail.com>]

On 15/12/2020 11:42, Germano Massullo wrote:
> Good day. I am investigating about a weird behavior of my system that
> leads to frequent freezes.
> The main components of my system are: Fedora 33 (kernel
> 5.9.13-200.fc33.x86_64), cryptsetup 2.3.4, an AMD Ryzen 5 3600, 16 GB
> RAM 3200 MHz, a Samsung 860 PRO used for 32 GB swap and LVM Cache, that
> is encrypted with following criteria:
> 
> # cryptsetup status ssd_cache
> /dev/mapper/ssd_cache is active and is in use.
>   type:    LUKS2
>   cipher:  aes-xts-plain64
>   keysize: 512 bits
>   key location: keyring
>   device:  /dev/sdc
>   sector size:  512
>   offset:  32768 sectors
>   size:    500085424 sectors
>   mode:    read/write
>   flags:   discards
> 
> During write bursts of swap (for example when booting some virtual
> machines), the system completely freezes, I cannot even move the mouse
> pointer. The system may remain in stuck state even for 10 minutes. When
> the (host) system turns back to normal, I can see for example in virtual
> machines, the following error in console
> ========
> Message from syslogd@localhost at Dec 15 01:14:32 ...
>  kernel:watchdog: BUG: soft lockup - CPU#0 stuck for 124s!
> [0x564a0d4baad0-:1924]
> ========
> 
> To check if the complete stuck of the system was caused by encryption, I
> tried to disable SSD swap, and create a unencrypted swap on a regular
> HDD, and then when I had write bursts, the system slowed, but it did not
> freeze.
> How could I proceed in my investigation to find out if it is a matter of
> system configuration, a bug, etc.?

First, this is kernel issue, not a cryptsetup userspace issue
(cryptsetup only configures kernel dm-crypt).

With some encryption drivers that allocates memory during encryption
there was a possible deadlock if swap is encrypted and system is out of memory.

There should be a workaround in latest stable kernel that does not allow
to use such crypto drivers in dm-crypt.
For dm-crypt it is commit
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers/md/dm-crypt.c?id=cd74693870fb748d812867ba49af733d689a3604
(but it requires also patches for crypto subsystem for setting this flag in drivers.)

The long-term solution is to fix crypto drivers...

Anyway, please try the latest stable kernel (ideally 5.10.x) - if you still
see the problem, please report it to dm-devel@redhat.com mailing list.

(There is another patch that limits IOs in flight in dm-crypt, but this was
not yet merged upstream - that could help too).

Thanks,
Milan
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