On Tue 10-12-19 18:43:03, Baoquan He wrote: > On 12/10/19 at 11:28am, Michal Hocko wrote: > > On Tue 10-12-19 15:24:53, Baoquan He wrote: [...] > > > But after system bootup, we should be able to hot add/remove any memory > > > board. This should not be restricted by a boot-time kernel parameter > > > 'mme='. This is what I am trying to fix. > > > > This is a simple statement without any actual explanation on why. Why is > > hotplug memory special? What is the usecase? Who would want to use mem > > parameter and later expect a memory above the restrected area to be > > hotplugable? > > The why is 'mem=' is used to restrict the amount of system ram during > boot. We have two ways to add system memory, one is installing DIMMs > before boot, the other is hot adding memory after boot. Without David's > use case, we may need redefine 'mem=' and change its documentation in > kernel-parameters.txt, if we don't want to fix it like this. Otherwise, > 'mem=' will limit the system's upper system ram always, that is not > expected. I really do not see why. It seems a pretty consistent behavior to me. Because it essentially cut any memory above the given size. If a new hotplugable memory fits into that cap then it just shows up. Quite contrary I would consider it unexpected that a memory higher than the given mem=XYZ is really there. But I do recognize a real usecase mentioned elsewhere which beats the consistency argument here because all setups where such a restriction would be really important are debugging/workaround AFAICS. -- Michal Hocko SUSE Labs
