From: SeongJae Park sjpark@amazon.de
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
--- drivers/net/xen-netback/xenbus.c | 2 ++ drivers/xen/xenbus/xenbus_client.c | 1 + drivers/xen/xenbus/xenbus_xs.c | 7 ++++++- include/xen/xenbus.h | 7 +++++++ 4 files changed, 16 insertions(+), 1 deletion(-)
--- a/drivers/net/xen-netback/xenbus.c +++ b/drivers/net/xen-netback/xenbus.c @@ -697,12 +697,14 @@ static int xen_register_watchers(struct 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; --- a/drivers/xen/xenbus/xenbus_client.c +++ b/drivers/xen/xenbus/xenbus_client.c @@ -120,6 +120,7 @@ int xenbus_watch_path(struct xenbus_devi int err;
watch->node = path; + watch->will_handle = NULL; watch->callback = callback;
err = register_xenbus_watch(watch); --- a/drivers/xen/xenbus/xenbus_xs.c +++ b/drivers/xen/xenbus/xenbus_xs.c @@ -903,7 +903,12 @@ static int process_msg(void) spin_lock(&watches_lock); msg->u.watch.handle = find_watch( msg->u.watch.vec[XS_WATCH_TOKEN]); - if (msg->u.watch.handle != NULL) { + if (msg->u.watch.handle != NULL && + (!msg->u.watch.handle->will_handle || + msg->u.watch.handle->will_handle( + msg->u.watch.handle, + (const char **)msg->u.watch.vec, + msg->u.watch.vec_size))) { spin_lock(&watch_events_lock); list_add_tail(&msg->list, &watch_events); wake_up(&watch_events_waitq); --- a/include/xen/xenbus.h +++ b/include/xen/xenbus.h @@ -58,6 +58,13 @@ 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 **vec, unsigned int len); + /* Callback (executed in a process context with no locks held). */ void (*callback)(struct xenbus_watch *, const char **vec, unsigned int len);