Introduce UserMemTotal and calculate MemAvailablePercent based on it

Shmem/tmpfs may block a large part of RAM. This memory cannot be freed
by killing processes. Before this change, we could get into a
situation where earlyoom kills *every* process, but still cannot
achieve the desired MemAvailablePercent.

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

main.c

@@ -341,8 +341,8 @@ int main(int argc, char* argv[])
     }
 
     // Print memory limits
-    fprintf(stderr, "mem total: %4lld MiB, swap total: %4lld MiB\n",
-        m.MemTotalKiB / 1024, m.SwapTotalKiB / 1024);
+    fprintf(stderr, "user mem total: %4lld MiB, swap total: %4lld MiB\n",
+        m.UserMemTotalKiB / 1024, m.SwapTotalKiB / 1024);
     fprintf(stderr, "sending SIGTERM when mem <= " PRIPCT " and swap <= " PRIPCT ",\n",
         args.mem_term_percent, args.swap_term_percent);
     fprintf(stderr, "        SIGKILL when mem <= " PRIPCT " and swap <= " PRIPCT "\n",
@@ -404,7 +404,7 @@ static unsigned sleep_time_ms(const poll_loop_args_t* args, const meminfo_t* m)
     const unsigned min_sleep = 100;
     const unsigned max_sleep = 1000;
 
-    long long mem_headroom_kib = (long long)((m->MemAvailablePercent - args->mem_term_percent) * (double)m->MemTotalKiB / 100);
+    long long mem_headroom_kib = (long long)((m->MemAvailablePercent - args->mem_term_percent) * (double)m->UserMemTotalKiB / 100);
     if (mem_headroom_kib < 0) {
         mem_headroom_kib = 0;
     }

meminfo.c

@@ -95,7 +95,7 @@ meminfo_t parse_meminfo()
     m.MemTotalKiB = get_entry_fatal("MemTotal:", buf);
     m.SwapTotalKiB = get_entry_fatal("SwapTotal:", buf);
     m.AnonPagesKiB = get_entry_fatal("AnonPages:", buf);
-    long long SwapFree = get_entry_fatal("SwapFree:", buf);
+    m.SwapFreeKiB = get_entry_fatal("SwapFree:", buf);
 
     m.MemAvailableKiB = get_entry("MemAvailable:", buf);
     if (m.MemAvailableKiB < 0) {
@@ -107,11 +107,13 @@ meminfo_t parse_meminfo()
         }
     }
 
+    // Calculated values
+    m.UserMemTotalKiB = m.MemAvailableKiB + m.AnonPagesKiB;
+
     // Calculate percentages
-    m.MemAvailablePercent = (double)m.MemAvailableKiB * 100 / (double)m.MemTotalKiB;
-    m.AnonPagesPercent = (double)m.AnonPagesKiB * 100 / (double)m.MemTotalKiB;
+    m.MemAvailablePercent = (double)m.MemAvailableKiB * 100 / (double)m.UserMemTotalKiB;
     if (m.SwapTotalKiB > 0) {
-        m.SwapFreePercent = (double)SwapFree * 100 / (double)m.SwapTotalKiB;
+        m.SwapFreePercent = (double)m.SwapFreeKiB * 100 / (double)m.SwapTotalKiB;
     } else {
         m.SwapFreePercent = 0;
     }
@@ -302,13 +304,11 @@ long long get_vm_rss_kib(int pid)
  */
 void print_mem_stats(int __attribute__((format(printf, 1, 2))) (*out_func)(const char* fmt, ...), const meminfo_t m)
 {
-    out_func("mem avail: %5lld of %5lld MiB (" PRIPCT "), swap free: %4lld of %4lld MiB (" PRIPCT "), anon: %5lld MiB (" PRIPCT ")\n",
+    out_func("mem avail: %5lld of %5lld MiB (" PRIPCT "), swap free: %4lld of %4lld MiB (" PRIPCT ")\n",
         m.MemAvailableKiB / 1024,
-        m.MemTotalKiB / 1024,
+        m.UserMemTotalKiB / 1024,
         m.MemAvailablePercent,
         m.SwapFreeKiB / 1024,
         m.SwapTotalKiB / 1024,
-        m.SwapFreePercent,
-        m.AnonPagesKiB / 1024,
-        m.AnonPagesPercent);
+        m.SwapFreePercent);
 }

meminfo.h

@@ -7,16 +7,19 @@
 #include <stdbool.h>
 
 typedef struct {
-    // Values from /proc/meminfo, in KiB or converted to MiB.
+    // Values from /proc/meminfo, in KiB
     long long MemTotalKiB;
     long long MemAvailableKiB;
     long long SwapTotalKiB;
     long long SwapFreeKiB;
     long long AnonPagesKiB;
+    // Calculated values
+    // UserMemTotalKiB = MemAvailableKiB + AnonPagesKiB.
+    // Represents the total amount of memory that may be used by user processes.
+    long long UserMemTotalKiB;
     // Calculated percentages
     double MemAvailablePercent; // percent of total memory that is available
     double SwapFreePercent; // percent of total swap that is free
-    double AnonPagesPercent; // percent of total memory that is used by processes
 } meminfo_t;
 
 typedef struct procinfo {