Re: [BUG] ZSwap leaks memory upon being disabled

[Konstantin Kharlamov <Hi-Angel@yandex.ru>]

On Sat, 2024-10-26 at 20:14 -0700, Nhat Pham wrote:
> On Sat, Oct 26, 2024 at 5:29 PM Konstantin Kharlamov
> <Hi-Angel@yandex.ru> wrote:
> > 
> > That was a good idea! The
> > `/sys/fs/cgroup/system.slice/memory.swap.current` seems to have the
> > missing half of the SWAP memory. From my understanding of the
> > `systemctl status` graph `sytem.slice` and `user.slice` groups do
> > not
> > intersect, and by adding up `system.slice/…` + `user.slice/…` I get
> > around 8G.
> > 
> > However, I'm still unclear what does this memory belong to.
> > `system.slice/memory.swap.current` is 4.4G currently, that's a lot
> > and
> > I'm not seeing anything that could take so much memory.
> 
> I assume you do not have any proactive memory reclaimer? :) 

No, just the kernel with `vm.swappiness = 100` and with ZSWAP (ZSWAP is
on on Archlinux nowadays via CONFIG_ZSWAP_DEFAULT_ON).

> I believe
> the top utility can display swap usage by process. Have you tried
> that?

I just tried. Well, the data seems the same as what `smem` shows,
except I can't add up the column numbers because top is interactive 😊
I noticed plasmashell was too bloated, so restarted it. Didn't solve
the problem with some unknown memory taking gigabytes in SWAP though.

> There are a couple of edge cases - for instance, if you disable zswap
> writeback and zswap at the same time. We will allocate slots on
> swapfile, and store it at the page table entry, but we cannot store
> the page's content in zswap or the swapfile, so the page remains in
> memory. You're occupying swap space, but are not really saving any
> memory usage.

I never disabled zswap writeback and as of writing the words zswap is
on, so this certainly not it.

> IIRC, there is also an edge case where a page is faulted back into
> memory from swap, but the associated swap space cannot be immediately
> released. This should be temporary though - memory reclaimer will
> attempt to release these pages later on, or they can be released when
> we scan the swapfile for slots during swap out.

Replied in a separate email.