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

On 19.03.21 13:36, David Hildenbrand wrote:

On 19.03.21 13:15, Muchun Song wrote:

quoted

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

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

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--- 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.

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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.

I think, right now, yes. System RAM that's applicable for HugePages
never uses an altmap. But Oscar's patch will change that, maybe before
your series might get included from what I've been reading. [1]

[1] https://lkml.kernel.org/r/20210319092635.6214-1-osalvador@suse.de

quoted

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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?

There has to be some way to identify whether we can optimize specific
vmemmap pages or should just leave them alone. altmap vs. !altmap.

Unfortunately, there is no easy way to detect that - e.g.,
PageReserved() applies also to boot memory.

We could go back to setting a special PageType for these vmemmap pages,
indicating "this is a page allocated from an altmap, don't touch it".

With SPARSEMEM we can use

PageReserved(page) && early_section(): vmemmap from bootmem

PageReserved(page) && !early_section(): vmemmap from altmap

!PageReserved(page): vmemmap from buddy

But it's a bit shaky :)

-- 
Thanks,

David / dhildenb