On 20.11.20 09:42, Michal Hocko wrote:
On Fri 20-11-20 14:43:04, Muchun Song wrote: [...] Thanks for improving the cover letter and providing some numbers. I have only glanced through the patchset because I didn't really have more time to dive depply into them. Overall it looks promissing. To summarize. I would prefer to not have the feature enablement controlled by compile time option and the kernel command line option should be opt-in. I also do not like that freeing the pool can trigger the oom killer or even shut the system down if no oom victim is eligible. One thing that I didn't really get to think hard about is what is the effect of vmemmap manipulation wrt pfn walkers. pfn_to_page can be invalid when racing with the split. How do we enforce that this won't blow up?
I have the same concerns - the sections are online the whole time and anybody with pfn_to_online_page() can grab them
I think we have similar issues with memory offlining when removing the vmemmap, it's just very hard to trigger and we can easily protect by grabbing the memhotplug lock. I once discussed with Dan using rcu to protect the SECTION_IS_ONLINE bit, to make sure anybody who did a pfn_to_online_page() stopped using the page. Of course, such an approach is not easy to use in this context where the sections stay online the whole time ... we would have to protect vmemmap table entries using rcu or similar, which can get quite ugly.
To keep things easy, maybe simply never allow to free these hugetlb pages again for now? If they were reserved during boot and the vmemmap condensed, then just let them stick around for all eternity.
Once we have a safe approach on how to modify an online vmemmap, we can enable this freeing, and eventually also dynamically manage vmemmaps for runtime-allocated huge pages.
-- Thanks, David / dhildenb
