On Tue, Mar 23, 2021 at 11:47:53AM +0100, Michal Hocko wrote: > On Fri 19-03-21 10:26:33, Oscar Salvador wrote: > > Self stored memmap leads to a sparse memory situation which is unsuitable > > for workloads that requires large contiguous memory chunks, so make this > > an opt-in which needs to be explicitly enabled. > > > > To control this, let memory_hotplug have its own memory space, as suggested > > by David, so we can add memory_hotplug.memmap_on_memory parameter. > > > > Signed-off-by: Oscar Salvador <osalvador@xxxxxxx> > > Reviewed-by: David Hildenbrand <david@xxxxxxxxxx> > > Acked-by: Michal Hocko <mhocko@xxxxxxxx> > > I would just rephrased the help text to be less low level ... > When enabled, runtime hotplugged memory will > allocate its internal metadata (struct pages) > from the hotadded memory which will allow to > hotadd a lot of memory without requiring > additional memory to do so. > This feature is disabled by default because it > has some implication on large (e.g. GB) > allocations in some configurations (e.g. small > memory blocks). Ok, this sounds good as well, and I guess it might suit best for what admin-guide is about. > The memmap_on_memory can be dropped from the 1st patch IIUC and only > introduce it now. It could be done, and I __think__ in some previous persion it was that way, but I am leaning to not do it. In the 1st patch, memmap_on_memory is false by default, so I see it as a preparatory step for later (this patchset) till it might be enabled. Moreover, the big comment from mhp_support_memmap_on_memory() should change to not mention it, and change here again to reflect it. All in all, I think it can stay, but maybe place a comment in the 1st patch above the variable saying something like "This is a noop now, it will be enabled later on" > > + > > +/* > > + * memory_hotplug.memmap_on_memory parameter > > + */ > > +static bool memmap_on_memory __ro_after_init; > > +#ifdef CONFIG_MHP_MEMMAP_ON_MEMORY > > +module_param(memmap_on_memory, bool, 0444); > > +MODULE_PARM_DESC(memmap_on_memory, "Enable memmap on memory for memory hotplug"); > > +#endif > > I am not very much familiar with the machinery. Does this expose the > state to the userspace? Kind of: # ls /sys/module/memory_hotplug/parameters memmap_on_memory # cat /sys/module/memory_hotplug/parameters/memmap_on_memory Y But that is not really the state, but rather it shows whether the user opted-in the feature by passing "memory_hotplug.memmap_on_memory=yes". It might be that the user opted-in the feature, but it cannot be used at at runtime (e.g: mhp_support_memmap_on_memory() return false due to size != memory_block_size()) -- Oscar Salvador SUSE L3
