Re: [PATCH v5] zswap: memcontrol: implement zswap writeback disabling

[Chris Li <chrisl@kernel.org>]

On Sat, Nov 18, 2023 at 11:23 AM Nhat Pham <nphamcs@gmail.com> wrote:
>
> Hmm how about this - in the future, we support the following
> options:
>
> 1. zswap.writeback == 1: no limitation to zswap writeback.
> All backing swap devices (sorted by priorities?) are fair game.
>
> 2. zswap.writeback == 0: disable all forms of zswap writeback.
>
> 3. zswap.writeback == <tiers description>:  attempt to write to each
> tier, one at a time.

We can merge the zswap.writeback as it is for now to unblock you.

For the future. I think we should remove zswap.writeback completely.

Instead we have:

swap.tiers == <swap_tier_list_name>
swap.tiers == "all" all available swap tiers. "zswap + swap file".
This is the default.
swap.tiers == "zswap" zswap only, no other swap file. Internally set
zswap.writeback = 0
swap.tiers == "foo" foo is a list of swap devices it can use. You can
define your town custom swap tier list in
swap.tiers == "none" or "disabled" Not allowed to swap.

"all", "zswap", "none" are reserved keywords.
"foo", "bar" etc are custom lists of swap tiers. User define custom
tier list in sys/kernel/mm/swap/tiers:
ssd:zswap,/dev/nvme01p4
hdd:/dev/sda4,/dev/sdb4

That would define two custom tiers. "ssd" can use zswap then /dev/nvme01p4.
The exact name of the "swap.tiers" and tiers name are open to suggestions.

>
> The first two are basically what we have for this patch.
> The last one will be added in a future patch.
>
> This is from the userspace perspective. Internally, we can modify
> memcg->writeback to be a pointer or a struct instead of this bool.
> (as you suggested).

Internally I would suggest memcg->swaptiers, the write back name is
somewhat confusing. As your patch indicated. It has two situation:
1. shrinking from zpool to real swapfile. The write back is appropriate here.
 2. zswap store failed (compression ratio too low, out of memory etc).
The write back is confusing here. It is more like writing through or
skip.

>
> This way, the API remains intact and backward compatible
> (and FWIW, I think there are still a lot of values in having simple
> options for the users who have simple memory hierarchies).

swap.tiers can be simple. For example, you can modify your patch to
"swap.tires == zswap" to
set zswap.writeback bool to 0 for now. Most of your patch is still re-usable.

I think we should discuss if we want to keep zswap.writeback in the
future because that would be some code undeletable and functionally
overlap with swap.tiers

Chris