Re: [External] Re: [PATCH v13 09/12] mm: hugetlb: add a kernel parameter hugetlb_free_vmemmap

[Muchun Song]

On Mon, Jan 25, 2021 at 7:43 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 17.01.21 16:10, Muchun Song wrote:
> > Add a kernel parameter hugetlb_free_vmemmap to enable the feature of
> > freeing unused vmemmap pages associated with each hugetlb page on boot.
>
> The description completely lacks a description of the changes performed
> in arch/x86/mm/init_64.c.

Will update. Thanks.

>
> [...]
>
> > --- a/arch/x86/mm/init_64.c
> > +++ b/arch/x86/mm/init_64.c
> > @@ -34,6 +34,7 @@
> >  #include <linux/gfp.h>
> >  #include <linux/kcore.h>
> >  #include <linux/bootmem_info.h>
> > +#include <linux/hugetlb.h>
> >
> >  #include <asm/processor.h>
> >  #include <asm/bios_ebda.h>
> > @@ -1557,7 +1558,8 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
> >  {
> >       int err;
> >
> > -     if (end - start < PAGES_PER_SECTION * sizeof(struct page))
> > +     if (is_hugetlb_free_vmemmap_enabled() ||
> > +         end - start < PAGES_PER_SECTION * sizeof(struct page))
>
> This looks irresponsible. You ignore any altmap, even though current
> altmap users (ZONE_DEVICE) will not actually result in applicable
> vmemmaps that huge pages could ever use.
>
> Why do you ignore the altmap completely? This has to be properly
> documented, but IMHO it's not even the right approach to mess with
> altmap here.

Thanks for reminding me of this. Sorry I also did not notice that.

>
> --
> Thanks,
>
> David / dhildenb
>