On Tue, Nov 12, 2019 at 10:21:23AM -0500, Johannes Weiner wrote: > 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. Okay, thanks! > 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. Right, that's certainly the design. I wonder if in the interests of defensively asserting requirements, it would be worth adding something like this to mem_cgroup_protected(): if (WARN_ON_ONCE(!memcg)) return MEMCG_PROT_NONE; ? -- Kees Cook
