Re: [PATCH v2 4/8] riscv: mm: Add memory hotplugging support

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On 14.05.24 16:04, Björn Töpel wrote:
From: Björn Töpel <bjorn@xxxxxxxxxxxx>

For an architecture to support memory hotplugging, a couple of
callbacks needs to be implemented:

  arch_add_memory()
   This callback is responsible for adding the physical memory into the
   direct map, and call into the memory hotplugging generic code via
   __add_pages() that adds the corresponding struct page entries, and
   updates the vmemmap mapping.

  arch_remove_memory()
   This is the inverse of the callback above.

  vmemmap_free()
   This function tears down the vmemmap mappings (if
   CONFIG_SPARSEMEM_VMEMMAP is enabled), and also deallocates the
   backing vmemmap pages. Note that for persistent memory, an
   alternative allocator for the backing pages can be used; The
   vmem_altmap. This means that when the backing pages are cleared,
   extra care is needed so that the correct deallocation method is
   used.

  arch_get_mappable_range()
   This functions returns the PA range that the direct map can map.
   Used by the MHP internals for sanity checks.

The page table unmap/teardown functions are heavily based on code from
the x86 tree. The same remove_pgd_mapping() function is used in both
vmemmap_free() and arch_remove_memory(), but in the latter function
the backing pages are not removed.

Signed-off-by: Björn Töpel <bjorn@xxxxxxxxxxxx>
---
  arch/riscv/mm/init.c | 242 +++++++++++++++++++++++++++++++++++++++++++
  1 file changed, 242 insertions(+)

diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index 6f72b0b2b854..7f0b921a3d3a 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -1493,3 +1493,245 @@ void __init pgtable_cache_init(void)
  	}
  }
  #endif
+
+#ifdef CONFIG_MEMORY_HOTPLUG
+static void __meminit free_pte_table(pte_t *pte_start, pmd_t *pmd)
+{
+	pte_t *pte;
+	int i;
+
+	for (i = 0; i < PTRS_PER_PTE; i++) {
+		pte = pte_start + i;
+		if (!pte_none(*pte))
+			return;
+	}
+
+	free_pages((unsigned long)page_address(pmd_page(*pmd)), 0);
+	pmd_clear(pmd);
+}
+
+static void __meminit free_pmd_table(pmd_t *pmd_start, pud_t *pud)
+{
+	pmd_t *pmd;
+	int i;
+
+	for (i = 0; i < PTRS_PER_PMD; i++) {
+		pmd = pmd_start + i;
+		if (!pmd_none(*pmd))
+			return;
+	}
+
+	free_pages((unsigned long)page_address(pud_page(*pud)), 0);
+	pud_clear(pud);
+}
+
+static void __meminit free_pud_table(pud_t *pud_start, p4d_t *p4d)
+{
+	pud_t *pud;
+	int i;
+
+	for (i = 0; i < PTRS_PER_PUD; i++) {
+		pud = pud_start + i;
+		if (!pud_none(*pud))
+			return;
+	}
+
+	free_pages((unsigned long)page_address(p4d_page(*p4d)), 0);
+	p4d_clear(p4d);
+}
+
+static void __meminit free_vmemmap_storage(struct page *page, size_t size,
+					   struct vmem_altmap *altmap)
+{
+	if (altmap)
+		vmem_altmap_free(altmap, size >> PAGE_SHIFT);
+	else
+		free_pages((unsigned long)page_address(page), get_order(size));

If you unplug a DIMM that was added during boot (can happen on x86-64, can it happen on riscv?), free_pages() would not be sufficient. You'd be freeing a PG_reserved page that has to be freed differently.

--
Cheers,

David / dhildenb





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