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 476849b0fba4 ("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 9f94db4c7eaa
("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.

Here is the log:
[    0.000000][    C0] ------------[ cut here ]------------
[    0.000000][    C0] WARNING: mm/sparse.c:365 at sparse_init+0x58a/0x6fe, CPU#0: swapper/0
[    0.000000][    C0] Modules linked in:
[    0.000000][    C0] CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted 7.2.0-rc3-g1d8304bdd65f #2 PREEMPT 
[    0.000000][    C0] Hardware name: riscv-virtio,qemu (DT)
[    0.000000][    C0] epc : sparse_init+0x58a/0x6fe
[    0.000000][    C0]  ra : sparse_init+0x58a/0x6fe
[    0.000000][    C0] epc : ffffffff86851c88 ra : ffffffff86851c88 sp : ffffffff88807a30
[    0.000000][    C0]  gp : ffffffff8a3bf240 tp : ffffffff88842080 t0 : ff600000ffab6000
[    0.000000][    C0]  t1 : 000000017fab6000 t2 : 65203a6573726363 s0 : ffffffff88807bc0
[    0.000000][    C0]  s1 : 000000000e000000 a0 : 0000000000000007 a1 : 0000000000000000
[    0.000000][    C0]  a2 : 0000000000000002 a3 : ffffffff86851c88 a4 : 0000000000000000
[    0.000000][    C0]  a5 : ffffffff88843080 a6 : 0000000000000003 a7 : 0000000000000000
[    0.000000][    C0]  s2 : ff60000000000000 s3 : 0040000000000000 s4 : 0004000000000000
[    0.000000][    C0]  s5 : ffffffff8a4d92e0 s6 : ff600000ffab55e0 s7 : ffffffff88384d00
[    0.000000][    C0]  s8 : 0000000000000003 s9 : ffffffff88384cc1 s10: ffffffff88384cc0
[    0.000000][    C0]  s11: ffffffff8a4daae0 t3 : ffffffff915e8b20 t4 : ffffffff915e8b20
[    0.000000][    C0]  t5 : ffffffff915e8b20 t6 : ffffffff915e8bc8 ssp : 0000000000000000
[    0.000000][    C0] status: 0000000200000100 badaddr: ffffffff86851c88 cause: 0000000000000003
[    0.000000][    C0] [<ffffffff86851c88>] sparse_init+0x58a/0x6fe
[    0.000000][    C0] [<ffffffff8683d396>] mm_core_init_early+0x116/0x1e30
[    0.000000][    C0] [<ffffffff86801edc>] start_kernel+0xd2/0x848

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.

Link: https://lore.kernel.org/20260716115326.3466926-1-xujiakai2025@iscas.ac.cn
Fixes: 476849b0fba4 ("riscv/mm: align vmemmap to maximal folio size")
Signed-off-by: Jiakai Xu <xujiakai2025@iscas.ac.cn>
Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexandre Ghiti <alex@ghiti.fr>
Cc: David Hildenbrand <david@kernel.org>
Cc: Guo Ren <guoren@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nam Cao <namcao@linutronix.de>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Assisted-by: YuanSheng:DeepSeek-V4-Flash
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index 3e450890be07..422efa11824b 100644
--- a/arch/riscv/mm/init.c
+++ b/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);