Re: phantom memory in a cgroup (was [BUG] ZSwap leaks memory upon being disabled)

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

On Mon, 2024-10-28 at 01:13 +0300, Konstantin Kharlamov wrote:
> On Sun, 2024-10-27 at 12:31 -0700, Yosry Ahmed wrote:
> > One thing you can do is take a snapshot of memory.stat when
> > memory.swap.current is at a high value (for sddm), then swapoff,
> > then
> > take another snapshot of memory.stat.
> >
> > We should see an increase in either anon or shmem, which will tell
> > us
> > which type of memory was swapped out.
>
> Okay. I will have to wait, because the session got killed by OOM. But
> I
> think it's gonna reproduce in just a few days, my new workflow seems
> to
> be triggering that a lot.

Done. I missed one cycle, which again got my session killed by OOM 😅
Now I caught this in time. The information was retrieved by:

    (systemctl status sddm && cat /sys/fs/cgroup/system.slice/sddm.service/memory.stat) > ~/Projects/cgroups-mem-leak/"$(date -R)".log

I wasn't sure how to represent it in email, and decided to post a diff
of "before `swapoff -a`" and "after …", to be viewed with `diffr` or
with `perl /path/to/diff-highlight` of git or similar.

Diff follows:

--- "Wed, 30 Oct 2024 17:27:38 +0300.log"       2024-10-30 17:27:38.401290017 +0300
+++ "Wed, 30 Oct 2024 17:28:12 +0300.log"       2024-10-30 17:28:12.397695798 +0300
@@ -6,8 +6,8 @@
              man:sddm.conf(5)
    Main PID: 710 (sddm)
       Tasks: 9 (limit: 18621)
-     Memory: 1.2G (peak: 2.8G swap: 1.7G swap peak: 3.2G zswap: 58.3M)
-        CPU: 6h 10min 7.847s
+     Memory: 2.8G (peak: 2.8G swap: 0B swap peak: 3.2G)
+        CPU: 6h 10min 14.748s
      CGroup: /system.slice/sddm.service
              ├─710 /usr/bin/sddm
              └─746 /usr/lib/Xorg -nolisten tcp -background none -seat seat0 vt2 -auth /run/sddm/xauth_GXHGRA -noreset -displayfd 20
@@ -22,36 +22,36 @@
 окт 28 09:22:36 dell-g15 sddm-helper[925]: Writing cookie to "/tmp/xauth_RKKlcB"
 окт 28 09:22:36 dell-g15 sddm-helper[925]: Starting X11 session: "" "/usr/share/sddm/scripts/Xsession \"env KDEWM=/usr/bin/i3 /usr/bin/startplasma-x11\""
 окт 28 09:22:36 dell-g15 sddm[710]: Session started true
-anon 42807296
-file 1150423040
-kernel 79376384
+anon 93822976
+file 2957750272
+kernel 18210816
 kernel_stack 147456
 pagetables 7204864
 sec_pagetables 0
 percpu 2184
 sock 0
 vmalloc 12288
-shmem 1150795776
-zswap 61173438
-zswapped 1751408640
+shmem 2958123008
+zswap 0
+zswapped 0
 file_mapped 4108288
 file_dirty 0
 file_writeback 0
-swapcached 17666048
+swapcached 0
 anon_thp 2097152
 file_thp 0
 shmem_thp 0
-inactive_anon 445014016
-active_anon 625201152
+inactive_anon 589209600
+active_anon 2321489920
 inactive_file 2895872
 active_file 2244608
-unevictable 140836864
-slab_reclaimable 8166656
-slab_unreclaimable 2618128
-slab 10784784
-workingset_refault_anon 69854
+unevictable 141144064
+slab_reclaimable 8169032
+slab_unreclaimable 2625208
+slab 10794240
+workingset_refault_anon 177253
 workingset_refault_file 12496
-workingset_activate_anon 33476
+workingset_activate_anon 41579
 workingset_activate_file 2372
 workingset_restore_anon 12558
 workingset_restore_file 2132
@@ -64,14 +64,14 @@
 pgsteal_kswapd 1243374
 pgsteal_direct 348876
 pgsteal_khugepaged 9149
-pgfault 626853
+pgfault 626941
 pgmajfault 11521
 pgrefill 560417
 pgactivate 85087
 pgdeactivate 0
 pglazyfree 0
 pglazyfreed 0
-zswpin 87568
+zswpin 515158
 zswpout 1395410
 zswpwb 211559
 thp_fault_alloc 8