On Wed 24-03-21 09:45:01, Oscar Salvador wrote: > 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" I will leave that up to you. This is likely not worth a larger discussion but it seems quite pointless to add a variable which never changes. The resulting code might look different than you expect because compiler is allowed to simply drop the whole condition. > > > + > > > +/* > > > + * 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()) Thanks for the clarification. -- Michal Hocko SUSE Labs
