Re: [PATCH v7] arm64: mm: Populate vmemmap at the page level if not section aligned

[David Hildenbrand <david@redhat.com>]

On 17.02.25 10:29, Zhenhua Huang wrote:
> On the arm64 platform with 4K base page config, SECTION_SIZE_BITS is set
> to 27, making one section 128M. The related page struct which vmemmap
> points to is 2M then.
> Commit c1cc1552616d ("arm64: MMU initialisation") optimizes the
> vmemmap to populate at the PMD section level which was suitable
> initially since hot plug granule is always one section(128M). However,
> commit ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
> introduced a 2M(SUBSECTION_SIZE) hot plug granule, which disrupted the
> existing arm64 assumptions.
> 
> The first problem is that if start or end is not aligned to a section
> boundary, such as when a subsection is hot added, populating the entire
> section is wasteful.
> 
> The Next problem is if we hotplug something that spans part of 128 MiB
> section (subsections, let's call it memblock1), and then hotplug something
> that spans another part of a 128 MiB section(subsections, let's call it
> memblock2), and subsequently unplug memblock1, vmemmap_free() will clear
> the entire PMD entry which also supports memblock2 even though memblock2
> is still active.
> 
> Assuming hotplug/unplug sizes are guaranteed to be symmetric. Do the
> fix similar to x86-64: populate to pages levels if start/end is not aligned
> with section boundary.
> 
> Signed-off-by: Zhenhua Huang <quic_zhenhuah@quicinc.com>
> ---
>   arch/arm64/mm/mmu.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index b4df5bc5b1b8..eec1666da368 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1178,7 +1178,8 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
>   {
>   	WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END));
>   
> -	if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES))
> +	if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES) ||
> +		(end - start < PAGES_PER_SECTION * sizeof(struct page)))
>   		return vmemmap_populate_basepages(start, end, node, altmap);
>   	else
>   		return vmemmap_populate_hugepages(start, end, node, altmap);

Yes, this does mimic what x86 does. That handling does look weird, because it
doesn't care about any address alignments, only about the size, which is odd.

I wonder if we could do better and move this handling
into vmemmap_populate_hugepages(), where we already have a fallback
to vmemmap_populate_basepages().

Something like:

One thing that confuses me is the "altmap" handling in x86-64 code: in particular
why it is ignored in some cases. So that might need a bit of thought / double-checking.


diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 01ea7c6df3036..57542313c0000 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -1546,10 +1546,10 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
         VM_BUG_ON(!PAGE_ALIGNED(start));
         VM_BUG_ON(!PAGE_ALIGNED(end));
  
-       if (end - start < PAGES_PER_SECTION * sizeof(struct page))
-               err = vmemmap_populate_basepages(start, end, node, NULL);
-       else if (boot_cpu_has(X86_FEATURE_PSE))
+       if (boot_cpu_has(X86_FEATURE_PSE))
                 err = vmemmap_populate_hugepages(start, end, node, altmap);
+       else
+               err = vmemmap_populate_basepages(start, end, node, NULL);
         else if (altmap) {
                 pr_err_once("%s: no cpu support for altmap allocations\n",
                                 __func__);
diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
index 3287ebadd167d..8b217265b25b1 100644
--- a/mm/sparse-vmemmap.c
+++ b/mm/sparse-vmemmap.c
@@ -300,6 +300,10 @@ int __weak __meminit vmemmap_check_pmd(pmd_t *pmd, int node,
         return 0;
  }
  
+/*
+ * Try to populate PMDs, but fallback to populating base pages when ranges
+ * would only partially cover a PMD.
+ */
  int __meminit vmemmap_populate_hugepages(unsigned long start, unsigned long end,
                                          int node, struct vmem_altmap *altmap)
  {
@@ -313,6 +317,9 @@ int __meminit vmemmap_populate_hugepages(unsigned long start, unsigned long end,
         for (addr = start; addr < end; addr = next) {
                 next = pmd_addr_end(addr, end);
  
+               if (!IS_ALIGNED(addr, PMD_SIZE) || !IS_ALIGNED(next, PMD_SIZE))
+                       goto fallback;
+
                 pgd = vmemmap_pgd_populate(addr, node);
                 if (!pgd)
                         return -ENOMEM;
@@ -346,6 +353,7 @@ int __meminit vmemmap_populate_hugepages(unsigned long start, unsigned long end,
                         }
                 } else if (vmemmap_check_pmd(pmd, node, addr, next))
                         continue;
+fallback:
                 if (vmemmap_populate_basepages(addr, next, node, altmap))
                         return -ENOMEM;
         }


-- 
Cheers,

David / dhildenb