On Wed 02-08-23 17:54:04, David Hildenbrand wrote: > On 02.08.23 17:50, Michal Hocko wrote: > > On Wed 02-08-23 10:15:04, Aneesh Kumar K V wrote: > > > On 8/1/23 4:20 PM, Michal Hocko wrote: > > > > On Tue 01-08-23 14:58:29, Aneesh Kumar K V wrote: > > > > > On 8/1/23 2:28 PM, Michal Hocko wrote: > > > > > > On Tue 01-08-23 10:11:16, Aneesh Kumar K.V wrote: > > > > > > > Allow updating memmap_on_memory mode after the kernel boot. Memory > > > > > > > hotplug done after the mode update will use the new mmemap_on_memory > > > > > > > value. > > > > > > > > > > > > Well, this is a user space kABI extension and as such you should spend > > > > > > more words about the usecase. Why we could live with this static and now > > > > > > need dynamic? > > > > > > > > > > > > > > > > This enables easy testing of memmap_on_memory feature without a kernel reboot. > > > > > > > > Testing alone is rather weak argument to be honest. > > > > > > > > > I also expect people wanting to use that when they find dax kmem memory online > > > > > failing because of struct page allocation failures[1]. User could reboot back with > > > > > memmap_on_memory=y kernel parameter. But being able to enable it via sysfs makes > > > > > the feature much more useful. > > > > > > > > Sure it can be useful but that holds for any feature, right. The main > > > > question is whether this is worth maintaing. The current implementation > > > > seems rather trivial which is an argument to have it but are there any > > > > risks long term? Have you evaluated a potential long term maintenance > > > > cost? There is no easy way to go back and disable it later on without > > > > breaking some userspace. > > > > > > > > All that should be in the changelog! > > > > > > I updated it as below. > > > > > > mm/memory_hotplug: Enable runtime update of memmap_on_memory parameter > > > > > > Allow updating memmap_on_memory mode after the kernel boot. Memory > > > hotplug done after the mode update will use the new mmemap_on_memory > > > value. > > > > > > It is now possible to test the memmap_on_memory feature easily without > > > the need for a kernel reboot. Additionally, for those encountering > > > struct page allocation failures while using dax kmem memory online, this > > > feature may prove useful. Instead of rebooting with the > > > memmap_on_memory=y kernel parameter, users can now enable it via sysfs, > > > which greatly enhances its usefulness. > > > > > > I do not really see a solid argument why rebooting is really a problem > > TBH. Also is the global policy knob really the right fit for existing > > hotplug usecases? In other words, if we really want to make > > memmap_on_memory more flexible would it make more sense to have it per > > memory block property instead (the global knob being the default if > > admin doesn't specify it differently). > > Per memory block isn't possible, due to the creation order. I am not sure I follow. Could you elaborate more? > Also, I think it's not the right approach. > > I thought about driver toggles. At least for dax/kmem people are looking > into that: > > https://lkml.kernel.org/r/20230801-vv-kmem_memmap-v3-2-406e9aaf5689@xxxxxxxxx > > Where that can also be toggled per device. Per device flag makes sense to me as well. But what we should do about hotplugged memory with a backing device (like regular RAM). I can see some reason to have large hotplugged memory ranges to be self vmemap hosted while smaller blocks to be backed by external vmemmaps. -- Michal Hocko SUSE Labs
