On Fri, Mar 19, 2021 at 4:59 PM Oscar Salvador [off-list ref] wrote:

On Mon, Mar 15, 2021 at 05:20:14PM +0800, Muchun Song wrote:

quoted

--- 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