vswitchd: Only lock pages that are faulted in.

The main purpose of locking the memory is to ensure that OVS can keep
doing what it did before in case of increased memory pressure, e.g.,
during VM ingest / migration.  Fulfilling this requirement can be
achieved without locking all the allocated memory, but only the pages
already accessed in the past (faulted in).  Processing of the new
traffic involves new memory allocations.  Latency on these operations
can't be guaranteed by the locking.  The main difference would be
the pre-faulting of the stack memory.  However, in order to revalidate
or process upcalls on the same traffic, the same amount of stack is
likely needed, so all the necessary memory will already be faulted in.

Switch 'mlockall' to MCL_ONFAULT to avoid consuming unnecessarily
large amounts of RAM on systems with high core counts.  For example,
in a densely populated OVN cluster this saves about 650 MB of RAM per
node on a system with 64 cores.  This equates to 320 GB of allocated
but unused RAM in a 500 node cluster.

This also makes OVS better suited by default for small systems with
limited amount of memory.

The MCL_ONFAULT flag was introduced in Linux kernel 4.4 and wasn't
available at the time of '--mlockall' introduction, but we can use it
now.  Falling back to an old way of locking in case we're running on
an older kernel just in case.

Only locking the faulted in pages also makes locking compatible with
vhost post-copy live migration by default, because we'll no longer
pre-fault all the guest's memory.  Post-copy relies on userfaultfd
to work on shared huge pages, which is only available in 4.11+ kernels.
So, technically, it should not be possible for MCL_ONFAULT to fail and
the call without it to succeed.  But keeping the check just in case
for now.

Acked-by: Simon Horman <horms@ovn.org>
Acked-by: Eelco Chaudron <echaudro@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

Documentation/ref/ovs-ctl.8.rst

@@ -170,8 +170,9 @@ The following options are less important:
 * ``--no-mlockall``
 
   By default ``ovs-ctl`` passes ``--mlockall`` to ``ovs-vswitchd``,
-  requesting that it lock all of its virtual memory, preventing it
-  from being paged to disk.  This option suppresses that behavior.
+  requesting that it lock all of its virtual memory on page fault (on
+  allocation, when running on Linux kernel 4.4 and older), preventing
+  it from being paged to disk.  This option suppresses that behavior.
 
 * ``--no-self-confinement``
 

Documentation/topics/dpdk/vhost-user.rst

@@ -340,8 +340,10 @@ The default value is ``false``.
     fixes (like userfaulfd leak) was released in 3.0.1.
 
     DPDK Post-copy feature requires avoiding to populate the guest memory
-    (application must not call mlock* syscall). So enabling mlockall is
-    incompatible with post-copy feature.
+    (application must not call mlock* syscall without MCL_ONFAULT).
+    So enabling mlockall is incompatible with post-copy feature in OVS 3.3 and
+    older. Newer versions of OVS only lock memory pages that are faulted in,
+    so both features can be used at the same time.
 
     Note that during migration of vhost-user device, PMD threads hang for the
     time of faulted pages download from source host. Transferring 1GB hugepage

NEWS

@@ -1,5 +1,7 @@
 Post-v3.3.0
 --------------------
+   - Option '--mlockall' now only locks memory pages on fault, if possible.
+     This also makes it compatible with vHost Post-copy Live Migration.
    - Userspace datapath:
      * Conntrack now supports 'random' flag for selecting ports in a range
        while natting and 'persistent' flag for selection of the IP address

lib/netdev-dpdk.c

@@ -6719,7 +6719,7 @@ parse_vhost_config(const struct smap *ovs_other_config)
 
     vhost_postcopy_enabled = smap_get_bool(ovs_other_config,
                                            "vhost-postcopy-support", false);
-    if (vhost_postcopy_enabled && memory_locked()) {
+    if (vhost_postcopy_enabled && memory_all_locked()) {
         VLOG_WARN("vhost-postcopy-support and mlockall are not compatible.");
         vhost_postcopy_enabled = false;
     }

lib/util.c

@@ -67,8 +67,8 @@ DEFINE_PER_THREAD_MALLOCED_DATA(char *, subprogram_name);
 /* --version option output. */
 static char *program_version;
 
-/* 'true' if mlockall() succeeded. */
-static bool is_memory_locked = false;
+/* 'true' if mlockall() succeeded, but doesn't support ONFAULT. */
+static bool is_all_memory_locked = false;
 
 /* Buffer used by ovs_strerror() and ovs_format_message(). */
 DEFINE_STATIC_PER_THREAD_DATA(struct { char s[128]; },
@@ -102,15 +102,15 @@ ovs_assert_failure(const char *where, const char *function,
 }
 
 void
-set_memory_locked(void)
+set_all_memory_locked(void)
 {
-    is_memory_locked = true;
+    is_all_memory_locked = true;
 }
 
 bool
-memory_locked(void)
+memory_all_locked(void)
 {
-    return is_memory_locked;
+    return is_all_memory_locked;
 }
 
 void

lib/util.h

@@ -156,8 +156,8 @@ void ctl_timeout_setup(unsigned int secs);
 
 void ovs_print_version(uint8_t min_ofp, uint8_t max_ofp);
 
-void set_memory_locked(void);
-bool memory_locked(void);
+void set_all_memory_locked(void);
+bool memory_all_locked(void);
 
 OVS_NO_RETURN void out_of_memory(void);
 

vswitchd/ovs-vswitchd.8.in

@@ -68,10 +68,11 @@ load the Open vSwitch kernel module.
 .PP
 .SH OPTIONS
 .IP "\fB\-\-mlockall\fR"
-Causes \fBovs\-vswitchd\fR to call the \fBmlockall()\fR function, to
-attempt to lock all of its process memory into physical RAM,
-preventing the kernel from paging any of its memory to disk.  This
-helps to avoid networking interruptions due to system memory pressure.
+Causes \fBovs\-vswitchd\fR to call the \fBmlockall()\fR function, to attempt to
+lock all of its process memory into physical RAM on page faults (on allocation,
+when running on Linux kernel 4.4 or older), preventing the kernel from paging
+any of its memory to disk.  This helps to avoid networking interruptions due to
+system memory pressure.
 .IP
 Some systems do not support \fBmlockall()\fR at all, and other systems
 only allow privileged users, such as the superuser, to use it.

vswitchd/ovs-vswitchd.c

@@ -56,7 +56,8 @@
 
 VLOG_DEFINE_THIS_MODULE(vswitchd);
 
-/* --mlockall: If set, locks all process memory into physical RAM, preventing
+/* --mlockall: If set, locks all present process memory pages into physical
+ * RAM and all the new pages the moment they are faulted in, preventing
  * the kernel from paging any of its memory to disk. */
 static bool want_mlockall;
 
@@ -96,10 +97,16 @@ main(int argc, char *argv[])
 
     if (want_mlockall) {
 #ifdef HAVE_MLOCKALL
-        if (mlockall(MCL_CURRENT | MCL_FUTURE)) {
-            VLOG_ERR("mlockall failed: %s", ovs_strerror(errno));
-        } else {
-            set_memory_locked();
+/* MCL_ONFAULT introduced in Linux kernel 4.4. */
+#ifndef MCL_ONFAULT
+#define MCL_ONFAULT 4
+#endif
+        if (mlockall(MCL_CURRENT | MCL_FUTURE | MCL_ONFAULT)) {
+            if (mlockall(MCL_CURRENT | MCL_FUTURE)) {
+                VLOG_ERR("mlockall failed: %s", ovs_strerror(errno));
+            } else {
+                set_all_memory_locked();
+            }
         }
 #else
         VLOG_ERR("mlockall not supported on this system");