xen/xenbus: Allow watches discard events before queueing

commit fed1755b118147721f2c87b37b9d66e62c39b668 upstream.

If handling logics of watch events are slower than the events enqueue
logic and the events can be created from the guests, the guests could
trigger memory pressure by intensively inducing the events, because it
will create a huge number of pending events that exhausting the memory.

Fortunately, some watch events could be ignored, depending on its
handler callback.  For example, if the callback has interest in only one
single path, the watch wouldn't want multiple pending events.  Or, some
watches could ignore events to same path.

To let such watches to volutarily help avoiding the memory pressure
situation, this commit introduces new watch callback, 'will_handle'.  If
it is not NULL, it will be called for each new event just before
enqueuing it.  Then, if the callback returns false, the event will be
discarded.  No watch is using the callback for now, though.

This is part of XSA-349

Cc: stable@vger.kernel.org
Signed-off-by: SeongJae Park <sjpark@amazon.de>
Reported-by: Michael Kurth <mku@amazon.de>
Reported-by: Pawel Wieczorkiewicz <wipawel@amazon.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/net/xen-netback/xenbus.c b/drivers/net/xen-netback/xenbus.c
index cd51492ae6c2dfae7bf9cef66b3206b8d126dbee..14273a431d99ea9e4b330743116f6173437802a7 100644 (file)
--- a/drivers/net/xen-netback/xenbus.c
+++ b/drivers/net/xen-netback/xenbus.c
@@ -777,12 +777,14 @@ static int xen_register_credit_watch(struct xenbus_device *dev,
                return -ENOMEM;
        snprintf(node, maxlen, "%s/rate", dev->nodename);
        vif->credit_watch.node = node;
+       vif->credit_watch.will_handle = NULL;
        vif->credit_watch.callback = xen_net_rate_changed;
        err = register_xenbus_watch(&vif->credit_watch);
        if (err) {
                pr_err("Failed to set watcher %s\n", vif->credit_watch.node);
                kfree(node);
                vif->credit_watch.node = NULL;
+               vif->credit_watch.will_handle = NULL;
                vif->credit_watch.callback = NULL;
        }
        return err;
@@ -829,6 +831,7 @@ static int xen_register_mcast_ctrl_watch(struct xenbus_device *dev,
        snprintf(node, maxlen, "%s/request-multicast-control",
                 dev->otherend);
        vif->mcast_ctrl_watch.node = node;
+       vif->mcast_ctrl_watch.will_handle = NULL;
        vif->mcast_ctrl_watch.callback = xen_mcast_ctrl_changed;
        err = register_xenbus_watch(&vif->mcast_ctrl_watch);
        if (err) {
@@ -836,6 +839,7 @@ static int xen_register_mcast_ctrl_watch(struct xenbus_device *dev,
                       vif->mcast_ctrl_watch.node);
                kfree(node);
                vif->mcast_ctrl_watch.node = NULL;
+               vif->mcast_ctrl_watch.will_handle = NULL;
                vif->mcast_ctrl_watch.callback = NULL;
        }
        return err;

diff --git a/drivers/xen/xenbus/xenbus_client.c b/drivers/xen/xenbus/xenbus_client.c
index f7b553faadb1014a2336cc363bd23dc8af09cb14..5a8bd3baa6e5d34d596141a492acd4fdec2d7034 100644 (file)
--- a/drivers/xen/xenbus/xenbus_client.c
+++ b/drivers/xen/xenbus/xenbus_client.c
@@ -120,6 +120,7 @@ int xenbus_watch_path(struct xenbus_device *dev, const char *path,
        int err;
 
        watch->node = path;
+       watch->will_handle = NULL;
        watch->callback = callback;
 
        err = register_xenbus_watch(watch);

diff --git a/drivers/xen/xenbus/xenbus_xs.c b/drivers/xen/xenbus/xenbus_xs.c
index 3a06eb699f33309f1d836235e6ba160a59c484fb..e8bdbd0a1e2669d57aac6a2a99d171718d67a077 100644 (file)
--- a/drivers/xen/xenbus/xenbus_xs.c
+++ b/drivers/xen/xenbus/xenbus_xs.c
@@ -705,7 +705,10 @@ int xs_watch_msg(struct xs_watch_event *event)
 
        spin_lock(&watches_lock);
        event->handle = find_watch(event->token);
-       if (event->handle != NULL) {
+       if (event->handle != NULL &&
+                       (!event->handle->will_handle ||
+                        event->handle->will_handle(event->handle,
+                                event->path, event->token))) {
                spin_lock(&watch_events_lock);
                list_add_tail(&event->list, &watch_events);
                wake_up(&watch_events_waitq);

diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h
index 869c816d5f8c3097b09298a9d086e7f75fff540e..55f543fe0bd8c48e4bfaf7b1288a4b715bdb6db2 100644 (file)
--- a/include/xen/xenbus.h
+++ b/include/xen/xenbus.h
@@ -59,6 +59,13 @@ struct xenbus_watch
        /* Path being watched. */
        const char *node;
 
+       /*
+        * Called just before enqueing new event while a spinlock is held.
+        * The event will be discarded if this callback returns false.
+        */
+       bool (*will_handle)(struct xenbus_watch *,
+                             const char *path, const char *token);
+
        /* Callback (executed in a process context with no locks held). */
        void (*callback)(struct xenbus_watch *,
                         const char *path, const char *token);