Re: [BUG] ZSwap leaks memory upon being disabled

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

On Sun, 2024-10-27 at 13:11 +0300, Konstantin Kharlamov wrote:
> On Sat, 2024-10-26 at 23:46 -0700, Yosry Ahmed wrote:
> > On Sat, Oct 26, 2024 at 8:14 PM Nhat Pham <nphamcs@gmail.com>
> > 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 am not very familiar with what usually runs in system.slice.
> >
> > >
> > > I assume you do not have any proactive memory reclaimer? :) I
> > > believe
> > > the top utility can display swap usage by process. Have you tried
> > > that?
> > >
> > > 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.
> > >
> > > 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.
> >
> > I don't think this is an edge case. I think when we swapin a page
> > we
> > generally leave it in the swapcache if there is no pressure on swap
> > space. In that case the memory is not really swapped out, but
> > because
> > it remains in the swapcache it is still reserving a swap slot, so
> > it
> > shows up as swap usage.
> >
> > Konstantin, could you check the amount of swapcache you have,
> > whether
> > through /proc/vmstat or memory.stat on both user and system slices?
>
> Sure
>
> 	λ grep cache /sys/fs/cgroup/*/memory.stat
> 	…
> 	/sys/fs/cgroup/system.slice/memory.stat:swapcached 434917376
> 	/sys/fs/cgroup/user.slice/memory.stat:swapcached 15478784
>
> `434917376` is a 0.4G, not much. In comparison,
> `system.slice/memory.swap.current` is currently `4764139520 = 4.4G`.

I figured since 434917376 is 10 numbers, I'd grep everything in
memory.stat that has ten digits:

    λ grep -P "\d{10}$" /sys/fs/cgroup/system.slice/memory.stat
    file 2671874048
    shmem 2592768000
    zswapped 2997760000
    active_anon 1491247104
    unevictable 1269555200

well, to me personally this isn't helpful, but perhaps am I missing
something…