oomd: Fix bug where we drop queued kill state on duplicate cgroup (#40690)

src/oom/oomd-manager.c

@@ -541,9 +541,8 @@ static int monitor_memory_pressure_contexts_handler(sd_event_source *s, uint64_t
                                 log_error_errno(r, "Failed to select any cgroups under %s based on pressure, ignoring: %m", t->path);
                         else {
                                 /* Don't act on all the high pressure cgroups at once; return as soon as we kill one.
-                                 * If r == 0 then it means there were not eligible candidates, the candidate cgroup
-                                 * disappeared, or the candidate cgroup has no processes by the time we tried to kill
-                                 * it. In either case, go through the event loop again and select a new candidate if
+                                 * If r == 0 then the cgroup is already queued for kill by an earlier iteration.
+                                 * In either case, go through the event loop again and select a new candidate if
                                  * pressure is still high. */
                                 m->mem_pressure_post_action_delay_start = usec_now;
                                 if (selected && r > 0) {

src/oom/oomd-util.c

@@ -494,12 +494,20 @@ int oomd_cgroup_kill_mark(Manager *m, OomdCGroupContext *ctx) {
         r = set_ensure_put(&m->kill_states, &oomd_kill_state_hash_ops, ks);
         if (r < 0)
                 return log_oom_debug();
+        if (r == 0) {
+                /* The cgroup is already queued. Drop only this temporary object, because running the normal
+                 * cleanup path would remove by cgroup path key and could interfere with the existing queued
+                 * kill state. */
+                oomd_cgroup_context_unref(ks->ctx);
+                ks = mfree(ks);
+                return 0;
+        }
 
         r = oomd_prekill_hook(m, ks);
         if (r < 0)
                 log_warning_errno(r, "oomd prekill hook failed for %s, ignoring: %m", ctx->path);
 
-        return 0;
+        return 1;
 }
 
 typedef void (*dump_candidate_func)(const OomdCGroupContext *ctx, FILE *f, const char *prefix);