zswap is not working with Linux Kernel 6.5.x

[Eugene San]

I recently migrated to Mantic from Jammy and noticed that 6.5.0 has "DEAD" zswap. It also didn't work with HWE-6.5.0 (both -9 and -14) on Jammy.

Everything seems normal, all the parameters and no error msgs but frontswap stays 0k at all times and everything gets swapped directly to disk.
No usage is reported in /sys/module/zswap/parameters and /sys/kernel/debug/zswap.

Jammy with HWE 6.2.0 and Mantic with https://kernel.ubuntu.com/mainline/v6.6.13/ and https://kernel.ubuntu.com/mainline/v6.7.1/ work fine.

I was able to reproduce the issue with https://kernel.ubuntu.com/mainline/v6.5.3/.
(I would try the latest https://kernel.ubuntu.com/mainline/v6.5.13/ but the build failed due building host issues.)

Seems like an upstream problem but not sure how to resolve it since Upstream 6.5 is abandoned and Jammy/Mantic 6.6 is months away...

[Matthew Ruffell (mruffell)]

Hi Eugene,

Can you try the latest 6.8-rc1 and let us know if it works there? Then we can see if we just need to backport a patch to 6.5, or talk to the upstream maintainers instead.

https://kernel.ubuntu.com/mainline/v6.8-rc1/

Thanks,
Matthew

[Eugene San (eugenesan)]

Hello Mathew,

Thanks for responding so promptly.

I did test 6.8-rc1 and ZSWAP works as expected.

But I've noticed something odd.
To quickly test ZSWAP, I use eatmemory tool to allocate 7GB of 8GB on my system.
Allocating worked and ZSWAP activated but freeing 7GB of memory took ~45s (system was barely responsive) instead of usual 1-2s.
I tested on freshly booted machine on native text console (no Graphics DE) and I didn't see that behavior on other kernels even with very busy system.
Looks like 6.8 needs some work or maybe there were some commits that altered swap/zswap behavior after 6.7.1.
So if you are planning on backporting patches to 6.5 I'd base those on 6.7.1 instead of 6.8.

I am back to using Noble's 6.6.0-14.14 which works ok for now.

P.S.
When testing mainline 6.6.13 I've had dead keyboard after suspend on my XPS13-9350, not sure if Nobles 6.6 has some XPS related fixes or I just was lucky to not catch the bug again ;-)
Please let me know if I need to open a separate bug for that.

Thanks,
Eugene.