riscv/mm: use physical alignment for vmemmap_start_pfn

RISC-V computes vmemmap_start_pfn by rounding phys_ram_base down to
VMEMMAP_ADDR_ALIGN. That alignment must therefore be expressed in the
physical-address domain.

Commit 476849b ("riscv/mm: align vmemmap to maximal folio size")
attempted to account for the maximal folio alignment by feeding
MAX_FOLIO_VMEMMAP_ALIGN directly into VMEMMAP_ADDR_ALIGN. However,
MAX_FOLIO_VMEMMAP_ALIGN is measured in bytes of struct page storage,
whereas VMEMMAP_ADDR_ALIGN is used to align a physical address.

The mask-based compound_info encoding requires pfn_to_page(0) to be
naturally aligned to MAX_FOLIO_VMEMMAP_ALIGN. Commit 9f94db4
("mm/sparse: check memmap alignment for compound_info_has_mask()")
added a check for that requirement and exposed the unit mismatch on
systems such as QEMU virt, where the DRAM base is not aligned to
MAX_FOLIO_NR_PAGES * PAGE_SIZE.

Convert MAX_FOLIO_VMEMMAP_ALIGN to the equivalent physical alignment
before using it in VMEMMAP_ADDR_ALIGN. This keeps the existing
round_down() logic while making the resulting vmemmap base satisfy the
mask-alignment requirement.

Fixes: 476849b ("riscv/mm: align vmemmap to maximal folio size")
Signed-off-by: Jiakai Xu <xujiakai2025@iscas.ac.cn>
Assisted-by: YuanSheng:DeepSeek-V4-Flash
Signed-off-by: Linux RISC-V bot <linux.riscv.bot@gmail.com>

Author: 6eanut

arch/riscv/mm/init.c

@@ -63,7 +63,8 @@ EXPORT_SYMBOL(phys_ram_base);
 
 #ifdef CONFIG_SPARSEMEM_VMEMMAP
 #define VMEMMAP_ADDR_ALIGN	max(1ULL << SECTION_SIZE_BITS, \
-				    MAX_FOLIO_VMEMMAP_ALIGN)
+				    PFN_PHYS(MAX_FOLIO_VMEMMAP_ALIGN / \
+					     sizeof(struct page)))
 
 unsigned long vmemmap_start_pfn __ro_after_init;
 EXPORT_SYMBOL(vmemmap_start_pfn);