On Wed 25-07-18 16:20:41, David Hildenbrand wrote: > On 25.07.2018 15:51, Michal Hocko wrote: > > On Tue 24-07-18 16:13:09, David Hildenbrand wrote: > > [...] > >> So I see right now: > >> > >> - Pg_reserved + e.g. new page type (or some other unique identifier in > >> combination with Pg_reserved) > >> -> Avoid reads of pages we know are offline > >> - extend is_ram_page() > >> -> Fake zero memory for pages we know are offline > >> > >> Or even both (avoid reading and don't crash the kernel if it is being done). > > > > I really fail to see how that can work without kernel being aware of > > PageOffline. What will/should happen if you run an old kdump tool on a > > kernel with this partially offline memory? > > > > New kernel with old dump tool: > > a) we have not fixed up is_ram_page() > > -> crash, as we access memory we shouldn't this is not acceptable, right? You do not want to crash your crash kernel ;) > b) we have fixed up is_ram_page() > > -> We have a callback to check for applicable memory in the hypervisor > whether the parts are accessible / online or not accessible / offline. > (e.g. via a device driver that controls a certain memory region) > > -> Don't read, but fake a page full of 0 > > > So instead of the kernel being aware of it, it asks via is_ram_page() > the hypervisor. I am still confused why do we even care about hypervisor. What if somebody wants to have partial memory hotplug on native OS? > I don't think a) is a problem. AFAICS, we have to update makedumpfile > for every new kernel. We can perform changes and update makedumpfile > to be compatible with new dump tools. Not really. You simply do not crash the kernel just because you are trying to dump the already crashed kernel. > E.g. remember SECTION_IS_ONLINE you introduced ? It broke dump > tools and required But has it crashed the kernel when reading the dump? If yes then the whole dumping is fragile as hell... -- Michal Hocko SUSE Labs
