Re: [PATCH v3 5/9] riscv: mm: Add memory hotplugging support

["Björn Töpel" <bjorn@kernel.org>]

Alexandre Ghiti <alexghiti@rivosinc.com> writes:

> On Tue, May 21, 2024 at 1:49 PM Björn Töpel <bjorn@kernel.org> wrote:
>>
>> From: Björn Töpel <bjorn@rivosinc.com>
>>
>> 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@rivosinc.com>
>> ---
>>  arch/riscv/mm/init.c | 261 +++++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 261 insertions(+)
>>
>> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
>> index 6f72b0b2b854..6693b742bf2f 100644
>> --- a/arch/riscv/mm/init.c
>> +++ b/arch/riscv/mm/init.c
>> @@ -1493,3 +1493,264 @@ void __init pgtable_cache_init(void)
>>         }
>>  }
>>  #endif
>> +
>> +#ifdef CONFIG_MEMORY_HOTPLUG
>> +static void __meminit free_pagetable(struct page *page, int order)
>> +{
>> +       unsigned int nr_pages = 1 << order;
>> +
>> +       /*
>> +        * vmemmap/direct page tables can be reserved, if added at
>> +        * boot.
>> +        */
>> +       if (PageReserved(page)) {
>> +               __ClearPageReserved(page);
>
> What's the difference between __ClearPageReserved() and
> ClearPageReserved()? Because it seems like free_reserved_page() calls
> the latter already, so why would you need to call
> __ClearPageReserved() on the first page?

Indeed! x86 copy pasta (which uses bootmem info page that RV doesn't).

>> +               while (nr_pages--)
>> +                       free_reserved_page(page++);
>> +               return;
>> +       }
>> +
>> +       free_pages((unsigned long)page_address(page), order);
>> +}
>> +
>> +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_pagetable(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_pagetable(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_pagetable(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_pagetable(page, get_order(size));
>> +}
>> +
>> +static void __meminit remove_pte_mapping(pte_t *pte_base, unsigned long addr, unsigned long end,
>> +                                        bool is_vmemmap, struct vmem_altmap *altmap)
>> +{
>> +       unsigned long next;
>> +       pte_t *ptep, pte;
>> +
>> +       for (; addr < end; addr = next) {
>> +               next = (addr + PAGE_SIZE) & PAGE_MASK;
>
> Nit: use ALIGN() instead.
>
>> +               if (next > end)
>> +                       next = end;
>> +
>> +               ptep = pte_base + pte_index(addr);
>> +               pte = READ_ONCE(*ptep);
>
> Nit: Use ptep_get()
>
>> +
>> +               if (!pte_present(*ptep))
>> +                       continue;
>> +
>> +               pte_clear(&init_mm, addr, ptep);
>> +               if (is_vmemmap)
>> +                       free_vmemmap_storage(pte_page(pte), PAGE_SIZE, altmap);
>> +       }
>> +}
>> +
>> +static void __meminit remove_pmd_mapping(pmd_t *pmd_base, unsigned long addr, unsigned long end,
>> +                                        bool is_vmemmap, struct vmem_altmap *altmap)
>> +{
>> +       unsigned long next;
>> +       pte_t *pte_base;
>> +       pmd_t *pmdp, pmd;
>> +
>> +       for (; addr < end; addr = next) {
>> +               next = pmd_addr_end(addr, end);
>> +               pmdp = pmd_base + pmd_index(addr);
>> +               pmd = READ_ONCE(*pmdp);
>
> Nit: Use pmdp_get()
>
>> +
>> +               if (!pmd_present(pmd))
>> +                       continue;
>> +
>> +               if (pmd_leaf(pmd)) {
>> +                       pmd_clear(pmdp);
>> +                       if (is_vmemmap)
>> +                               free_vmemmap_storage(pmd_page(pmd), PMD_SIZE, altmap);
>> +                       continue;
>> +               }
>> +
>> +               pte_base = (pte_t *)pmd_page_vaddr(*pmdp);
>> +               remove_pte_mapping(pte_base, addr, next, is_vmemmap, altmap);
>> +               free_pte_table(pte_base, pmdp);
>> +       }
>> +}
>> +
>> +static void __meminit remove_pud_mapping(pud_t *pud_base, unsigned long addr, unsigned long end,
>> +                                        bool is_vmemmap, struct vmem_altmap *altmap)
>> +{
>> +       unsigned long next;
>> +       pud_t *pudp, pud;
>> +       pmd_t *pmd_base;
>> +
>> +       for (; addr < end; addr = next) {
>> +               next = pud_addr_end(addr, end);
>> +               pudp = pud_base + pud_index(addr);
>> +               pud = READ_ONCE(*pudp);
>
> Nit: Use pudp_get()
>
>> +
>> +               if (!pud_present(pud))
>> +                       continue;
>> +
>> +               if (pud_leaf(pud)) {
>> +                       if (pgtable_l4_enabled) {
>> +                               pud_clear(pudp);
>> +                               if (is_vmemmap)
>> +                                       free_vmemmap_storage(pud_page(pud), PUD_SIZE, altmap);
>> +                       }
>> +                       continue;
>> +               }
>> +
>> +               pmd_base = pmd_offset(pudp, 0);
>> +               remove_pmd_mapping(pmd_base, addr, next, is_vmemmap, altmap);
>> +
>> +               if (pgtable_l4_enabled)
>> +                       free_pmd_table(pmd_base, pudp);
>> +       }
>> +}
>> +
>> +static void __meminit remove_p4d_mapping(p4d_t *p4d_base, unsigned long addr, unsigned long end,
>> +                                        bool is_vmemmap, struct vmem_altmap *altmap)
>> +{
>> +       unsigned long next;
>> +       p4d_t *p4dp, p4d;
>> +       pud_t *pud_base;
>> +
>> +       for (; addr < end; addr = next) {
>> +               next = p4d_addr_end(addr, end);
>> +               p4dp = p4d_base + p4d_index(addr);
>> +               p4d = READ_ONCE(*p4dp);
>
> Nit: Use p4dp_get()

...and I'll make sure to address these nits as well.


Thanks!
Björn