oomd: Fix Killed signal reason being lost

Emitting "oom" doesn't mesh with the org.freedesktop.oom1.Manager
Killed() API contract, which defines "memory-used" and "memory-pressure"
as possible reasons. Consumers that key off reason thus will either lose
policy attribution or may reject the unknown value completely.

Plumb the reason through so it is visible to consumers.

src/oom/oomd-manager.c

@@ -416,7 +416,7 @@ static int monitor_swap_contexts_handler(sd_event_source *s, uint64_t usec, void
                         return 0;
                 }
 
-                r = oomd_cgroup_kill_mark(m, selected);
+                r = oomd_cgroup_kill_mark(m, selected, "memory-used");
                 if (r == -ENOMEM)
                         return log_oom();
                 if (r < 0)
@@ -534,7 +534,7 @@ static int monitor_memory_pressure_contexts_handler(sd_event_source *s, uint64_t
                                 return 0;
                         }
 
-                        r = oomd_cgroup_kill_mark(m, selected);
+                        r = oomd_cgroup_kill_mark(m, selected, "memory-pressure");
                         if (r == -ENOMEM)
                                 return log_oom();
                         if (r < 0)

src/oom/oomd-util.c

@@ -27,6 +27,7 @@
 typedef struct OomdKillState {
         Manager *manager;
         OomdCGroupContext *ctx;
+        const char *reason;
         /* This holds sd_varlink references */
         Set *links;
 } OomdKillState;
@@ -245,11 +246,12 @@ int oomd_sort_cgroup_contexts(Hashmap *h, oomd_compare_t compare_func, const cha
         return (int) k;
 }
 
-int oomd_cgroup_kill(Manager *m, OomdCGroupContext *ctx, bool recurse) {
+int oomd_cgroup_kill(Manager *m, OomdCGroupContext *ctx, bool recurse, const char *reason) {
         _cleanup_set_free_ Set *pids_killed = NULL;
         int r;
 
         assert(ctx);
+        assert(!m || reason);
 
         pids_killed = set_new(NULL);
         if (!pids_killed)
@@ -288,7 +290,7 @@ int oomd_cgroup_kill(Manager *m, OomdCGroupContext *ctx, bool recurse) {
                                           "Killed",
                                           "ss",
                                           ctx->path,
-                                          "oom");
+                                          reason);
 
         return !set_isempty(pids_killed);
 }
@@ -334,7 +336,7 @@ static void oomd_kill_state_remove(OomdKillState *ks) {
         if (!set_isempty(ks->links))
                 return;
 
-        r = oomd_cgroup_kill(ks->manager, ks->ctx, /* recurse= */ true);
+        r = oomd_cgroup_kill(ks->manager, ks->ctx, /* recurse= */ true, ks->reason);
         if (r < 0)
                 log_debug_errno(r, "Failed to kill cgroup '%s', ignoring: %m", ks->ctx->path);
         oomd_kill_state_free(ks);
@@ -465,11 +467,12 @@ static int oomd_prekill_hook(Manager *m, OomdKillState *ks) {
         return 0;
 }
 
-int oomd_cgroup_kill_mark(Manager *m, OomdCGroupContext *ctx) {
+int oomd_cgroup_kill_mark(Manager *m, OomdCGroupContext *ctx, const char *reason) {
         int r;
 
         assert(ctx);
         assert(m);
+        assert(reason);
 
         if (m->dry_run) {
                 _cleanup_free_ char *cg_path = NULL;
@@ -489,6 +492,7 @@ int oomd_cgroup_kill_mark(Manager *m, OomdCGroupContext *ctx) {
         *ks = (OomdKillState) {
                 .manager = m,
                 .ctx = oomd_cgroup_context_ref(ctx),
+                .reason = reason,
         };
 
         r = set_ensure_put(&m->kill_states, &oomd_kill_state_hash_ops, ks);

src/oom/oomd-util.h

@@ -124,8 +124,8 @@ int oomd_sort_cgroup_contexts(Hashmap *h, oomd_compare_t compare_func, const cha
 int oomd_fetch_cgroup_oom_preference(OomdCGroupContext *ctx, const char *prefix);
 
 /* Returns a negative value on error, 0 if no processes were killed, or 1 if processes were killed. */
-int oomd_cgroup_kill(Manager *m, OomdCGroupContext *ctx, bool recurse);
-int oomd_cgroup_kill_mark(Manager *m, OomdCGroupContext *ctx);
+int oomd_cgroup_kill(Manager *m, OomdCGroupContext *ctx, bool recurse, const char *reason);
+int oomd_cgroup_kill_mark(Manager *m, OomdCGroupContext *ctx, const char *reason);
 
 /* The following oomd_kill_by_* functions return 1 if processes were killed, or negative otherwise. */
 /* If `prefix` is supplied, only cgroups whose paths start with `prefix` are eligible candidates. Otherwise,

src/oom/test-oomd-util.c

@@ -88,7 +88,11 @@ TEST(oomd_cgroup_kill) {
                 ASSERT_OK(fork_and_sleep(5, &two));
                 ASSERT_OK(cg_attach(subcgroup, two.pid));
 
-                ASSERT_OK_POSITIVE(oomd_cgroup_kill(NULL /* manager */, &(OomdCGroupContext){ .path = subcgroup }, false /* recurse */));
+                ASSERT_OK_POSITIVE(oomd_cgroup_kill(
+                                                   /* m= */ NULL,
+                                                   &(OomdCGroupContext){ .path = subcgroup },
+                                                   /* recurse= */ false,
+                                                   /* reason= */ NULL));
 
                 ASSERT_OK(cg_get_xattr(subcgroup, "user.oomd_ooms", &v, /* ret_size= */ NULL));
                 ASSERT_STREQ(v, i == 0 ? "1" : "2");