Re: [RFC PATCH v2 0/4] mm/swap: reserve swap areas for deliberate offload

[Matthias Goergens]

Hi Chris,

Thanks for resolving the conflicts yourself and reviewing it.

> What is the high-level user-visible impact of this series?

Being able to use more interesting swap backends and logic when the
machine is not under memory pressure.  It is much easier to write a
swap backend that may occasionally allocate memory than one that is
guaranteed never to, so today such backends are either unsafe as swap
or ruled out (btrfs, for example, refuses swapfiles that are
copy-on-write, checksummed or compressed).

> I am curious: if we never run out of swap file space on the
> non-offload swap area, does that mean we don't need this patch series?

No: running out of conventional swap isn't the point.  Without the
series, any active swap area may be written under pressure, so a
backend that may allocate can't be used as swap at all, however much
conventional swap there is next to it.

> However, direct reclaim can use both types of swap areas.

In v2 it can't: only memory.reclaim, per-node reclaim and MGLRU's
debugfs eviction may write new data to an offload-only area; direct
reclaim, kswapd, MADV_PAGEOUT and DAMON reclaim may not.  But I think
your suggestion to flip it round is closer to what I want: mark the
areas whose writes may allocate, and keep pressure reclaim away from
those, rather than tying it to what started the reclaim.  I'll work
that into v3, together with Kairui's suggestion to build on the swap
tiers work.

Thanks,
Matthias