Re: [External] Re: [PATCH v19 7/8] mm: hugetlb: add a kernel parameter hugetlb_free_vmemmap

[Muchun Song <songmuchun@bytedance.com>]

On Fri, Mar 19, 2021 at 4:59 PM Oscar Salvador <osalvador@suse.de> wrote:
>
> On Mon, Mar 15, 2021 at 05:20:14PM +0800, Muchun Song wrote:
> > --- 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()  && !altmap) ||
> > +         end - start < PAGES_PER_SECTION * sizeof(struct page))
> >               err = vmemmap_populate_basepages(start, end, node, NULL);
> >       else if (boot_cpu_has(X86_FEATURE_PSE))
> >               err = vmemmap_populate_hugepages(start, end, node, altmap);
>
> I've been thinking about this some more.
>
> Assume you opt-in the hugetlb-vmemmap feature, and assume you pass a valid altmap
> to vmemmap_populate.
> This will lead to use populating the vmemmap array with hugepages.

Right.

>
> What if then, a HugeTLB gets allocated and falls within that memory range (backed
> by hugetpages)?

I am not sure whether we can allocate the HugeTLB pages from there.
Will only device memory pass a valid altmap parameter to
vmemmap_populate()? If yes, can we allocate HugeTLB pages from
device memory? Sorry, I am not an expert on this.


> AFAIK, this will get us in trouble as currently the code can only operate on memory
> backed by PAGE_SIZE pages, right?
>
> I cannot remember, but I do not think nothing prevents that from happening?
> Am I missing anything?

Maybe David H is more familiar with this.

Hi David,

Do you have some suggestions on this?

Thanks.


>
> --
> Oscar Salvador
> SUSE L3