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xen/xenbus: Allow watches discard events before queueing

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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>
This commit is contained in:
SeongJae Park 2020-12-14 10:02:45 +01:00 committed by Greg Kroah-Hartman
parent aadd67750f
commit b4898bdd20
4 changed files with 16 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
drivers/net/xen-netback/xenbus.c
View File

@ -557,12 +557,14 @@ 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;
@ -609,6 +611,7 @@ 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) {
@ -616,6 +619,7 @@ 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;

1
drivers/xen/xenbus/xenbus_client.c
View File

@ -133,6 +133,7 @@ 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);

5
drivers/xen/xenbus/xenbus_xs.c
View File

@ -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);

7
include/xen/xenbus.h
View File

@ -61,6 +61,13 @@ 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);