On 05/02/2012 05:03 AM, Andrew Morton wrote:
On Thu, 19 Apr 2012 16:54:50 +0800
Sha Zhengju<handai.szj@xxxxxxxxx> wrote:
From: Sha Zhengju<handai.szj@xxxxxxxxxx>
When the last event is unregistered, there is no need to keep the spare
array anymore. So free it to avoid memory leak.
How serious is this leak? Is there any way in which it can be used to
consume unbounded amounts of memory?
While registering events, the ->primary will apply for a larger array to
store
the new threshold info and the ->spare holds the old primary space.
But once unregistering event, the ->primary and ->spare pointer will be
swapped
after updating thresholds info. So if we have an eventfd with many(>1)
thresholds
attached to it, mem_cgroup_usage_unregister_event() will finally leave
->spare
holding a large array and have no chance to be freed.
I hope it is clear.
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4412,6 +4412,12 @@ static void mem_cgroup_usage_unregister_event(struct cgroup *cgrp,
swap_buffers:
/* Swap primary and spare array */
thresholds->spare = thresholds->primary;
+ /* If all events are unregistered, free the spare array */
+ if (!new) {
+ kfree(thresholds->spare);
+ thresholds->spare = NULL;
+ }
+
rcu_assign_pointer(thresholds->primary, new);
The resulting code is really quite convoluted. Try to read through it
and follow the handling of ->primary and ->spare. Head spins.
What is the protocol here? If ->primary is NULL then ->spare must also
be NULL?
To be simple: if new(->primary) is NULL, it means we are unregistering
the last threshold and there is no need to keep ->spare any more.
So give the ->spare array a chance to be freed.
Thanks,
Sha
I'll apply the patch, although I don't (yet) have sufficient info to
know which kernels it should be applied to. Perhaps someone could
revisit this code and see if it can be made more straightforward.
.
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