From 92d4ced9881361aefa2f7bc58dab19aa4d28ddf8 Mon Sep 17 00:00:00 2001 From: SeongJae Park Date: Mon, 14 Dec 2020 10:02:45 +0100 Subject: [PATCH] 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 Reported-by: Michael Kurth Reported-by: Pawel Wieczorkiewicz Reviewed-by: Juergen Gross Signed-off-by: Juergen Gross Signed-off-by: Greg Kroah-Hartman --- drivers/net/xen-netback/xenbus.c | 4 ++++ drivers/xen/xenbus/xenbus_client.c | 1 + drivers/xen/xenbus/xenbus_xs.c | 5 ++++- include/xen/xenbus.h | 7 +++++++ 4 files changed, 16 insertions(+), 1 deletion(-) diff --git a/drivers/net/xen-netback/xenbus.c b/drivers/net/xen-netback/xenbus.c index a56d3eab35dd6..e46276f8e4a5d 100644 --- 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 f7b553faadb10..5a8bd3baa6e5d 100644 --- 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 b609c6e08796e..63f1d3fd4d5b4 100644 --- a/drivers/xen/xenbus/xenbus_xs.c +++ b/drivers/xen/xenbus/xenbus_xs.c @@ -700,7 +700,10 @@ 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 869c816d5f8c3..55f543fe0bd8c 100644 --- 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);