On Sat, Nov 21, 2020 at 1:47 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote: > > On 11/20/20 1:43 AM, David Hildenbrand wrote: > > On 20.11.20 10:39, Michal Hocko wrote: > >> On Fri 20-11-20 10:27:05, David Hildenbrand wrote: > >>> 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 am not sure we can/want to span memory hotplug locking out to all pfn > >> walkers. But you are right that the underlying problem is similar but > >> much harder to trigger because vmemmaps are only removed when the > >> physical memory is hotremoved and that happens very seldom. Maybe it > >> will happen more with virtualization usecases. But this work makes it > >> even more tricky. If a pfn walker races with a hotremove then it would > >> just blow up when accessing the unmapped physical address space. For > >> this feature a pfn walker would just grab a real struct page re-used for > >> some unpredictable use under its feet. Any failure would be silent and > >> hard to debug. > > > > Right, we don't want the memory hotplug locking, thus discussions regarding rcu. Luckily, for now I never saw a BUG report regarding this - maybe because the time between memory offlining (offline_pages()) and memory/vmemmap getting removed (try_remove_memory()) is just too long. Someone would have to sleep after pfn_to_online_page() for quite a while to trigger it. > > > >> > >> [...] > >>> 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. > >> > >> Not sure I understand. Do you propose to only free those vmemmap pages > >> when the pool is initialized during boot time and never allow to free > >> them up? That would certainly make it safer and maybe even simpler wrt > >> implementation. > > > > Exactly, let's keep it simple for now. I guess most use cases of this (virtualization, databases, ...) will allocate hugepages during boot and never free them. > > Not sure if I agree with that last statement. Database and virtualization > use cases from my employer allocate allocate hugetlb pages after boot. It > is shortly after boot, but still not from boot/kernel command line. > > Somewhat related, but not exactly addressing this issue ... > > One idea discussed in a previous patch set was to disable PMD/huge page > mapping of vmemmap if this feature was enabled. This would eliminate a bunch > of the complex code doing page table manipulation. It does not address > the issue of struct page pages going away which is being discussed here, > but it could be a way to simply the first version of this code. If this > is going to be an 'opt in' feature as previously suggested, then eliminating > the PMD/huge page vmemmap mapping may be acceptable. My guess is that > sysadmins would only 'opt in' if they expect most of system memory to be used > by hugetlb pages. We certainly have database and virtualization use cases > where this is true. Hi Mike, Yeah, I agree with you that the first version of this feature should be simply. I can do that (disable PMD/huge page mapping of vmemmap) in the next version patch. But I have another question: what the problem is when struct page pages go away? I have not understood the issues discussed here, hope you can answer for me. Thanks. > -- > Mike Kravetz -- Yours, Muchun
