Enable zstd backend for zram

[okulev]

What would you like to be added:

Please enable possibility to use zstd backend for zram in Linux 6.12. It was possible in Linux 6.6: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1086172#14

Why is this needed:

"Zstd would most likely be a much better suited default, as it's a great compromise between speed and efficiency, and users can then choose on their own if they rather want to use e.g. LZO or LZ4 instead." -- from https://bugs.debian.org/1086172

What ended up in Debian is https://salsa.debian.org/kernel-team/linux/-/commit/121c3f1335d97b6938d76e7541d9c4579e83d69c . I would recommend also adding CONFIG_ZRAM_DEF_COMP_ZSTD=y , as mentioned in the Debian bug.

[pnpavlov]

@gehoern please confirm you are OK with this change, then the change will be executed as part of next release planned for Feb

[gehoern]

We should extend this request a bit:

• CONFIG_ZRAM_BACKEND_LZO=y
  adds also lzo compression (the algorithm is already in the kernel, so not much of an overhead)
• CONFIG_ZRAM_DEF_COMP_ZSTD=y
  CONFIG_ZRAM_DEF_COMP="zstd"

The current default is lz4 which is in my opinion not optimal:

• the best compression ratio is (depending on circumstances) zstd
• the fastest (lowest cpu usage) is lzo

so i support this switch to zstd. And we should also do this on 6.6 and 6.12 line, since we are extending functionality (lzo) and switching the default standard (zstd) and the zram disk is anyhow gone after reboot, so no issues expected. (btw. not many of our customers are using zram disks, but since we have this request, i might be wrong.)

[fwilhe]

@gehoern added the LZO option

zstd set as standard is already in the PR https://github.com/gardenlinux/package-linux/pull/67/files

kindly review and approve if you think it's good

thank you

[pnpavlov]

@okulev , the change is merged in the 6.12 kernel with #67 and will be available with next Garden Linux major new release (17xx). For the existing releases (1443 & 1592) with 6.6 kernel, the change is prepared in #68 and can be included with next patches after approval from @gehoern .

Could you please confirm the change is what you expected and reach out to me directly over chat / email if there is certain timeline that you would like to meet.

[okulev]

Thank you! Later I noticed that Debian also restored CONFIG_ZRAM_BACKEND_DEFLATE=y in https://salsa.debian.org/kernel-team/linux/-/commit/1bbc3e9821bf97c7e059184ace6acd6fb6c824e2 . The only reason I see for also adding DEFLATE is for alignment with Debian kernel - as compression algorithm I don't see any advantages to zstd (for zram).

I use today dist kernel on Debian Stable for several personal servers, so I can validate it works as expected on next 6.12 release.