Re: [PATCH v5 3/5] mm,memory_hotplug: Add kernel boot option to enable memmap_on_memory

[Oscar Salvador <osalvador@suse.de>]

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@suse.de>
> > Reviewed-by: David Hildenbrand <david@redhat.com>
> 
> Acked-by: Michal Hocko <mhocko@suse.com>
> 
> 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