Re: [PATCH v3] mm/memory_hotplug: refrain from adding memory into an impossible node

On Thu, 2020-04-16 at 08:19 +0200, Michal Hocko wrote: > On Wed 15-04-20 20:32:00, Verma, Vishal L wrote: >>> >>> I really do not like this. Why should we try to be clever and change the >>> node id requested by the caller? I would just stick with node_possible >>> check and be done with this. >> >> Hi Michal, >> >> Being clever allows us to still use the memory even if it is in a non- >> optimal configuration. Failing here leaves the user no path to add this >> memory until the firmware is fixed. It is the tradeoff between some >> usability vs. how loud we want to be for the failure. > > Doing that papers over something that is clearly a FW issue and makes > it "my performance is suboptimal" deal with it OS problem. Really, is > this something we have to care about. Your changelog talks about a Qemu > misconfiguration which is trivial to fix. Has this ever been observed > with a real HW? > Well - more of a qemu bug I think - I can share the details, but it just looked like it was producing a bogus SRAT. I think it is plausible that such a firmware bug can happen out in the wild. The NFIT tables would just need to reference a 'proximity domain' that the SRAT hasn't previously described, and hotplug will happily go add memory from the NFIT and the backing node related data structures would be missing. I'm not too opposed to erroring out, so long as we are ok with the fact that we will leave some memory stranded until there's a firmware fix.