Support Linux's oom_score_adj API as well as the older oom_adj API.

The simplest way to handle this is just to copy-and-paste the relevant
code block in fork_process.c, so that's what I did. (It's possible that
something more complicated would be useful to packagers who want to work
with either the old or the new API; but at this point the number of such
people is rapidly approaching zero, so let's just get the minimal thing
done.)  Update relevant documentation as well.

contrib/start-scripts/linux

@@ -42,10 +42,14 @@ PGLOG="$PGDATA/serverlog"
 
 # It's often a good idea to protect the postmaster from being killed by the
 # OOM killer (which will tend to preferentially kill the postmaster because
-# of the way it accounts for shared memory).  Setting the OOM_ADJ value to
+# of the way it accounts for shared memory).  Setting the OOM_SCORE_ADJ value
-# -17 will disable OOM kill altogether.  If you enable this, you probably want
+# to -1000 will disable OOM kill altogether.  If you enable this, you probably
-# to compile PostgreSQL with "-DLINUX_OOM_ADJ=0", so that individual backends
+# want to compile PostgreSQL with "-DLINUX_OOM_SCORE_ADJ=0", so that
-# can still be killed by the OOM killer.
+# individual backends can still be killed by the OOM killer.
+#OOM_SCORE_ADJ=-1000
+# Older Linux kernels may not have /proc/self/oom_score_adj, but instead
+# /proc/self/oom_adj, which works similarly except the disable value is -17.
+# For such a system, enable this and compile with "-DLINUX_OOM_ADJ=0".
 #OOM_ADJ=-17
 
 ## STOP EDITING HERE
@@ -78,6 +82,7 @@ test -x $DAEMON ||
 case $1 in
   start)
 	echo -n "Starting PostgreSQL: "
+	test x"$OOM_SCORE_ADJ" != x && echo "$OOM_SCORE_ADJ" > /proc/self/oom_score_adj
 	test x"$OOM_ADJ" != x && echo "$OOM_ADJ" > /proc/self/oom_adj
 	su - $PGUSER -c "$DAEMON -D '$PGDATA' &" >>$PGLOG 2>&1
 	echo "ok"
@@ -90,6 +95,7 @@ case $1 in
   restart)
 	echo -n "Restarting PostgreSQL: "
 	su - $PGUSER -c "$PGCTL stop -D '$PGDATA' -s -m fast -w"
+	test x"$OOM_SCORE_ADJ" != x && echo "$OOM_SCORE_ADJ" > /proc/self/oom_score_adj
 	test x"$OOM_ADJ" != x && echo "$OOM_ADJ" > /proc/self/oom_adj
 	su - $PGUSER -c "$DAEMON -D '$PGDATA' &" >>$PGLOG 2>&1
 	echo "ok"

doc/src/sgml/runtime.sgml

@@ -1268,7 +1268,7 @@ default:\
     In Linux 2.4 and later, the default virtual memory behavior is not
     optimal for <productname>PostgreSQL</productname>. Because of the
     way that the kernel implements memory overcommit, the kernel might
-    terminate the <productname>PostgreSQL</productname> server (the
+    terminate the <productname>PostgreSQL</productname> postmaster (the
     master server process) if the memory demands of
     another process cause the system to run out of virtual memory.
    </para>
@@ -1317,22 +1317,31 @@ sysctl -w vm.overcommit_memory=2
    <para>
     Another approach, which can be used with or without altering
     <varname>vm.overcommit_memory</>, is to set the process-specific
-    <varname>oom_adj</> value for the postmaster process to <literal>-17</>,
+    <varname>oom_score_adj</> value for the postmaster process to
-    thereby guaranteeing it will not be targeted by the OOM killer.  The
+    <literal>-1000</>, thereby guaranteeing it will not be targeted by the OOM
-    simplest way to do this is to execute
+    killer.  The simplest way to do this is to execute
 <programlisting>
-echo -17 > /proc/self/oom_adj
+echo -1000 > /proc/self/oom_score_adj
 </programlisting>
     in the postmaster's startup script just before invoking the postmaster.
     Note that this action must be done as root, or it will have no effect;
     so a root-owned startup script is the easiest place to do it.  If you
     do this, you may also wish to build <productname>PostgreSQL</>
-    with <literal>-DLINUX_OOM_ADJ=0</> added to <varname>CPPFLAGS</>.
+    with <literal>-DLINUX_OOM_SCORE_ADJ=0</> added to <varname>CPPFLAGS</>.
     That will cause postmaster child processes to run with the normal
-    <varname>oom_adj</> value of zero, so that the OOM killer can still
+    <varname>oom_score_adj</> value of zero, so that the OOM killer can still
     target them at need.
    </para>
 
+   <para>
+    Older Linux kernels do not offer <filename>/proc/self/oom_score_adj</>,
+    but may have a previous version of the same functionality called
+    <filename>/proc/self/oom_adj</>.  This works the same except the disable
+    value is <literal>-17</> not <literal>-1000</>.  The corresponding
+    build flag for <productname>PostgreSQL</> is
+    <literal>-DLINUX_OOM_ADJ=0</>.
+   </para>
+
    <note>
    <para>
     Some vendors' Linux 2.4 kernels are reported to have early versions

src/backend/postmaster/fork_process.c

@@ -68,12 +68,40 @@ fork_process(void)
 		 * process sizes *including shared memory*.  (This is unbelievably
 		 * stupid, but the kernel hackers seem uninterested in improving it.)
 		 * Therefore it's often a good idea to protect the postmaster by
-		 * setting its oom_adj value negative (which has to be done in a
+		 * setting its oom_score_adj value negative (which has to be done in a
 		 * root-owned startup script). If you just do that much, all child
 		 * processes will also be protected against OOM kill, which might not
-		 * be desirable.  You can then choose to build with LINUX_OOM_ADJ
+		 * be desirable.  You can then choose to build with
-		 * #defined to 0, or some other value that you want child processes to
+		 * LINUX_OOM_SCORE_ADJ #defined to 0, or to some other value that you
-		 * adopt here.
+		 * want child processes to adopt here.
+		 */
+#ifdef LINUX_OOM_SCORE_ADJ
+		{
+			/*
+			 * Use open() not stdio, to ensure we control the open flags. Some
+			 * Linux security environments reject anything but O_WRONLY.
+			 */
+			int			fd = open("/proc/self/oom_score_adj", O_WRONLY, 0);
+
+			/* We ignore all errors */
+			if (fd >= 0)
+			{
+				char		buf[16];
+				int			rc;
+
+				snprintf(buf, sizeof(buf), "%d\n", LINUX_OOM_SCORE_ADJ);
+				rc = write(fd, buf, strlen(buf));
+				(void) rc;
+				close(fd);
+			}
+		}
+#endif   /* LINUX_OOM_SCORE_ADJ */
+
+		/*
+		 * Older Linux kernels have oom_adj not oom_score_adj.  This works
+		 * similarly except with a different scale of adjustment values.
+		 * If it's necessary to build Postgres to work with either API,
+		 * you can define both LINUX_OOM_SCORE_ADJ and LINUX_OOM_ADJ.
 		 */
 #ifdef LINUX_OOM_ADJ
 		{