Don't kill anything if there's not enough anon memory used

When /dev/shm and /tmp (when on tmpfs) gets filled up, earlyoom
would start killing everything, without improving the situation.

Check if we *possibly* could kill enough processes (i.e. anon memory)
to get the available memory over the minimum. If not, do nothing.

Fixes: #274
Fixes: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1012870

main.c

@@ -421,11 +421,30 @@ static unsigned sleep_time_ms(const poll_loop_args_t* args, const meminfo_t* m)
  */
 static int lowmem_sig(const poll_loop_args_t* args, const meminfo_t* m)
 {
+    int sig = 0;
     if (m->MemAvailablePercent <= args->mem_kill_percent && m->SwapFreePercent <= args->swap_kill_percent)
-        return SIGKILL;
+        sig = SIGKILL;
     else if (m->MemAvailablePercent <= args->mem_term_percent && m->SwapFreePercent <= args->swap_term_percent)
-        return SIGTERM;
-    return 0;
+        sig = SIGTERM;
+    else
+        return 0;
+
+    long long want_kib = (long long)((double)m->SwapTotalKiB * args->swap_term_percent / 100 + (double)m->MemTotalKiB * args->mem_term_percent / 100);
+    long long have_kib = m->MemAvailableKiB + m->SwapFreeMiB * 1024;
+    // how much do we have to free by killing processes?
+    long long to_free_kib = want_kib - have_kib;
+    debug("%s: want_kib=%lld have_kib=%lld to_free_kib=%lld\n", __func__, want_kib, have_kib, to_free_kib);
+
+    // Let's not kill *everything* when the user has filled up /dev/shm and /tmp (when on tmpfs).
+    if (to_free_kib >= m->AnonPagesKiB) {
+        warn("We need to free %lld MiB, but there's only %lld MiB of anon memory that we could free!\n",
+            to_free_kib / 1024, m->AnonPagesKiB / 1024);
+        warn("Nothing we can do here. Sleeping 1 second.\n");
+        sleep(1);
+        return 0;
+    }
+
+    return sig;
 }
 
 // poll_loop is the main event loop. Never returns.