On Wed, 26 Jul 2017 14:30:41 +0200 Michal Hocko <mhocko@xxxxxxxxxx> wrote: > On Wed 26-07-17 13:45:39, Heiko Carstens wrote: > [...] > > In general I do like your idea, however if I understand your patches > > correctly we might have an ordering problem on s390: it is not possible to > > access hot-added memory on s390 before it is online (MEM_GOING_ONLINE > > succeeded). > > Could you point me to the code please? I cannot seem to find the > notifier which implements that. It is in drivers/s390/char/sclp_cmd.c: sclp_mem_notifier(). > > > On MEM_GOING_ONLINE we ask the hypervisor to back the potential available > > hot-added memory region with physical pages. Accessing those ranges before > > that will result in an exception. > > Can we make the range which backs the memmap range available? E.g from > s390 specific __vmemmap_populate path? No, only the complete range of a storage increment can be made available. The size of those increments may vary between z/VM and LPAR, but at least with LPAR it will always be minimum 256 MB, IIRC. > > > However with your approach the memory is still allocated when add_memory() > > is being called, correct? That wouldn't be a change to the current > > behaviour; except for the ordering problem outlined above. > > Could you be more specific please? I do not change when the memmap is > allocated. I guess this is about the difference between s390 and others, wrt when we call add_memory(). It is also in drivers/s390/char/sclp_cmd.c, early during memory detection, as opposed to other archs, where I guess this could be triggered by an ACPI event during runtime, at least for newly added and to-be-onlined memory. This probably means that any approach that tries to allocate memmap memory during add_memory(), out of the "to-be-onlined but still offline" memory, will be difficult for s390, because we call add_memory() only once during memory detection for the complete range of (hypervisor) defined online and offline memory. The offline parts are then made available in the MEM_GOING_ONLINE notifier called from online_pages(). Only after this point the memory would then be available to allocate a memmap in it. Nevertheless, we have great interest in such a "allocate memmap from the added memory range" solution. I guess we would need some way to separate the memmap allocation from add_memory(), which sounds odd, or provide some way to have add_memory() only allocate a memmap for online memory, and a mechanism to add the memmaps for offline memory blocks later when they are being set online. Regards, Gerald -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
