Subject: Re: Coverity: shrink_node_memcgs(): Null pointer dereferences

From: Johannes Weiner <[email-masked]>
Date: Tue, 12 Nov 2019 10:21:23 -0500

On Mon, Nov 11, 2019 at 05:35:37PM -0800, coverity-bot wrote:
> Hello!
> 
> This is an experimental automated report about issues detected by Coverity
> from a scan of next-20191108 as part of the linux-next weekly scan project:
> https://scan.coverity.com/projects/linux-next-weekly-scan
> 
> You're getting this email because you were associated with the identified
> lines of code (noted below) that were touched by recent commits:
> 
> c34aa3085f94 ("mm-vmscan-split-shrink_node-into-node-part-and-memcgs-part-fix")
> 
> Coverity reported the following:
> 
> *** CID 1487844:  Null pointer dereferences  (NULL_RETURNS)
> /mm/vmscan.c: 2695 in shrink_node_memcgs()
> 2689     	memcg = mem_cgroup_iter(target_memcg, NULL, NULL);
> 2690     	do {
> 2691     		struct lruvec *lruvec = mem_cgroup_lruvec(memcg, pgdat);
> 2692     		unsigned long reclaimed;
> 2693     		unsigned long scanned;
> 2694
> vvv     CID 1487844:  Null pointer dereferences  (NULL_RETURNS)
> vvv     Dereferencing a pointer that might be "NULL" "memcg" when calling "mem_cgroup_protected".
> 2695     		switch (mem_cgroup_protected(target_memcg, memcg)) {

This appears to be a false alarm.

All the "culprit" patch did was rename the local variable
"target_memcg".

And while it's correct that memcg can be NULL (befor and after this
patch), it's the case only when mem_cgroup_disabled(), and
mem_cgroup_protected() checks for this case.