* Vlastimil Babka [off-list ref] [2020-03-17 17:45:15]:

On 3/17/20 5:25 PM, Srikar Dronamraju wrote:

quoted

* Vlastimil Babka [off-list ref] [2020-03-17 16:56:04]:

quoted

I wonder why do you get a memory leak while Sachin in the same situation [1]
gets a crash? I don't understand anything anymore.

Sachin was testing on linux-next which has Kirill's patch which modifies
slub to use kmalloc_node instead of kmalloc. While Bharata is testing on
upstream, which doesn't have this. 

Yes, that Kirill's patch was about the memcg shrinker map allocation. But the
patch hunk that Bharata posted as a "hack" that fixes the problem, it follows
that there has to be something else that calls kmalloc_node(node) where node is
one that doesn't have present pages.

He mentions alloc_fair_sched_group() which has:

        for_each_possible_cpu(i) {
                cfs_rq = kzalloc_node(sizeof(struct cfs_rq),
                                      GFP_KERNEL, cpu_to_node(i));
...
                se = kzalloc_node(sizeof(struct sched_entity),
                                  GFP_KERNEL, cpu_to_node(i));

Sachin's experiment.
Upstream-next/ memcg /
possible nodes were 0-31
online nodes were 0-1
kmalloc_node called for_each_node / for_each_possible_node.
This would crash while allocating slab from !N_ONLINE nodes.

Bharata's experiment.
Upstream
possible nodes were 0-1
online nodes were 0-1
kmalloc_node called for_each_online_node/ for_each_possible_cpu
i.e kmalloc is called for N_ONLINE nodes.
So wouldn't crash

Even if his possible nodes were 0-256. I don't think we have kmalloc_node
being called in !N_ONLINE nodes. Hence its not crashing.
If we see the above code that you quote, kzalloc_node is using cpu_to_node
which in Bharata's case will always return 1.

I assume one of these structs is 1k and other 512 bytes (rounded) and that for
some possible cpu's cpu_to_node(i) will be 0, which has no present pages. And as
Bharata pasted, node_to_mem_node(0) = 0
So this looks like the same scenario, but it doesn't crash? Is the node 0
actually online here, and/or does it have N_NORMAL_MEMORY state?

I still dont have any clue on the leak though.

-- 
Thanks and Regards
Srikar Dronamraju