Re: [External] Re: [PATCH v20 8/9] mm: memory_hotplug: disable memmap_on_memory when hugetlb_free_vmemmap enabled

[Muchun Song]

On Wed, Apr 21, 2021 at 3:33 PM Oscar Salvador <osalvador@xxxxxxx> wrote:
>
> On Wed, Apr 21, 2021 at 11:41:24AM +0800, Muchun Song wrote:
> > > Documentation/admin-guide/kernel-parameters.txt already provides an
> > > explanation on memory_hotplug.memmap_on_memory parameter that states
> > > that the feature cannot be enabled when using hugetlb-vmemmap
> > > optimization.
> > >
> > > Users can always check whether the feature is enabled via
> > > /sys/modules/memory_hotplug/parameters/memmap_on_memory.
>
> Heh, I realized this is not completely true.
> Users can check whether the feature is __enabled__ by checking the sys fs,
> but although it is enabled, it might not be effective.

Right. I have done the test.

>
> This might be due to a different number of reasons, vmemmap does not fully
> span a PMD, the size we want to add spans more than a single memory block, etc.

Agree. Thanks for your explanations.

>
> That is what
>
> "Note that even when enabled, there are a few cases where the feature is not
>  effective."
>
> is supposed to mean.
>
> Anyway, I did not change my opionion on this.
>
> Thanks
>
> --
> Oscar Salvador
> SUSE L3