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. Making the necessary changes to support runtime updates is a simple change that should not affect the addition of new features or result in long-term maintenance problems.
