jail: Try disabling Transparent Hugepage on Linux

Disabling Transparent Hugepage has often been the solution to solve
hard-to-diagnose instability issues and despite improvements in this
area compared to the RHEL6 era, our recommandation is still to avoid
THP to this day.

In addition to refreshing the documentation on this topic, the Linux
jail will tentatively disable THP or warn when it cannot.

bin/varnishd/mgt/mgt_jail_linux.c

@@ -94,19 +94,38 @@ vjl_make_workdir(const char *dname, const char *what, struct vsb *vsb)
 
 	vjl_master(JAIL_MASTER_FILE);
 	if (statfs(dname, &info) != 0) {
-		if (vsb)
-			VSB_printf(vsb, "Could not stat working directory '%s': %s (%d)\n", dname, VAS_errtxt(errno), errno);
-		else
-			MGT_Complain(C_ERR, "Could not stat working directory '%s': %s (%d)", dname, VAS_errtxt(errno), errno);
+		if (vsb) {
+			VSB_printf(vsb,
+			    "Could not stat working directory '%s': %s (%d)\n",
+			    dname, VAS_errtxt(errno), errno);
+		} else {
+			MGT_Complain(C_ERR,
+			    "Could not stat working directory '%s': %s (%d)",
+			    dname, VAS_errtxt(errno), errno);
+		}
 		return (1);
 	}
 	if (info.f_type != TMPFS_MAGIC) {
 		if (vsb != NULL)
-			VSB_printf(vsb, "Working directory not mounted on tmpfs partition\n");
+			VSB_printf(vsb,
+			    "Working directory not mounted on tmpfs partition\n");
 		else
-			MGT_Complain(C_INFO, "Working directory not mounted on tmpfs partition");
+			MGT_Complain(C_INFO,
+			    "Working directory not mounted on tmpfs partition");
 	}
 	vjl_master(JAIL_MASTER_LOW);
+
+	if (prctl(PR_SET_THP_DISABLE, 1, 0, 0, 0) != 0) {
+		if (vsb != NULL) {
+			VSB_printf(vsb,
+			    "Could not disable Transparent Hugepage: %s (%d)\n",
+			    VAS_errtxt(errno), errno);
+		} else {
+			MGT_Complain(C_INFO,
+			    "Could not disable Transparent Hugepage: %s (%d)",
+			    VAS_errtxt(errno), errno);
+		}
+	}
 	return (0);
 }
 

doc/sphinx/installation/platformnotes.rst

@@ -11,21 +11,29 @@ On some platforms it is necessary to adjust the operating system before running
 Varnish on it. The systems and steps known to us are described in this section.
 
 
-Transparent hugepages on Redhat Enterprise Linux 6
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Transparent Hugepage on Linux
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-On RHEL6 Transparent Hugepage kernel support is enabled by default.
-This is known to cause sporadic crashes of Varnish.
+On certain Linux distributions Transparent Hugepage kernel support is enabled
+by default. This is known to cause instability of Varnish.
 
-It is recommended to disable transparent hugepages on affected
-systems. This can be done with
-``echo never > /sys/kernel/mm/redhat_transparent_hugepage/enabled``
-(runtime) or by adding "transparent_hugepage=never" to the kernel boot
-line in the "/etc/grub.conf" file (persistent).
+It is recommended to disable Transparent Hugepage on affected systems.
+If Varnish is the only significant service running on this system, this can be
+done during runtime with::
 
-On Debian/Ubuntu systems running 3.2 kernels the default value is "madvise" and
-does not need to be changed.
+  echo never > /sys/kernel/mm/transparent_hugepage/enabled
 
+Alternatively, this can be persisted in your bootloader configuration by adding
+``transparent_hugepage=never`` to the kernel command line.
+
+On other systems the default value is ``madvise`` and does not need to be
+changed. Either way, the Linux :ref:`ref-varnishd-opt_j` will try to disable
+Transparent Hugepage in the ``varnishd`` process.
+
+The general recommendation is to mount the working directory in a ``tmpfs``
+partition, usually ``/var/lib/varnish``. By default tmpfs should be mounted with
+Transparent Hugepage disabled. Consider mounting the working directory with the
+``huge=never`` mount option if that is not the case with your OS vendor.
 
 OpenVZ
 ~~~~~~

doc/sphinx/reference/varnishd.rst

@@ -461,6 +461,7 @@ specific options. Available jails are:
 
   - It warns when *workdir* is not on a ``tmpfs``.
   - It tries to keep the process dumpable after dropping privileges.
+  - It tries to disable Transparent Hugepage in ``varnishd``
 
 -j <unix[,user=`user`][,ccgroup=`group`][,workuser=`user`]>