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

[Kurt Fitzner <kurt_cryptsetup@va1der.ca>]

On 2025-11-27 13:54, Mikulas Patocka wrote:

> Encrypted swap file is not supposed to work.

Do you have a reference for this?  The concept of encrypted swap files 
has been a valid workflow for a very long time.

> So, this is what happened to you - the machine runs out of memory, it
> needs to swap out some pages, dm-crypt encrypts the pages and generates
> write bios, the write bios are directed to the loop device, the loop
> device directs them to the filesystem, the filesystem attempts to 
> allocate
> more memory => deadlock.

If it's the filesystem trying to allocate memory on writes to a swap 
file that is causing a memory allocation/swap race, then any write to a 
swap file would engender the same result, regardless of encryption. The 
encryption layer is redundant under the failure mode you propose.

I can confirm I have put kernels up to and including 6.14 under heavy 
memory stress and have never encountered anything that feels like a 
memory allocation race.  All my systems have encrypted swap files.

I can't speak toward later kernels, or any bugs that may or may not be 
presesnt, but I know of nothing to suggest that encrypted swap files 
remain anything other than an intended feature.

     Kurt