[PATCH v2] riscv: mm: Fix alignment of phys_ram_base

[Xu Lu]

This commit fixes the alignment of phys_ram_base in RISC-V.

In sparse vmemmap model, the virtual address of vmemmap is calculated as:
((struct page *)VMEMMAP_START - (phys_ram_base >> PAGE_SHIFT)).
And the struct page's va can be calculated with an offset:
(vmemmap + (pfn)).

However, when initializing struct pages, kernel actually starts from the
first page from the same section that phys_ram_base belongs to. If the
first page's physical address is not (phys_ram_base >> PAGE_SHIFT), then
we get an va below VMEMMAP_START when calculating va for it's struct page.

For example, if phys_ram_base starts from 0x82000000 with pfn 0x82000, the
first page in the same section is actually pfn 0x80000. During
init_unavailable_range(), we will initialize struct page for pfn 0x80000
with virtual address ((struct page *)VMEMMAP_START - 0x2000), which is
below VMEMMAP_START as well as PCI_IO_END.

This commit fixes this bug by aligning phys_ram_base with SECTION_SIZE.

Fixes: c3bcc65d4d2e ("riscv: Start of DRAM should at least be aligned on PMD size for the direct mapping")
Signed-off-by: Xu Lu <luxu.kernel at bytedance.com>
---
 arch/riscv/mm/init.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index 0e8c20adcd98..974cafa7c85e 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -33,6 +33,9 @@
 #include <asm/pgtable.h>
 #include <asm/sections.h>
 #include <asm/soc.h>
+#ifdef CONFIG_SPARSEMEM
+#include <asm/sparsemem.h>
+#endif
 #include <asm/tlbflush.h>
 
 #include "../kernel/head.h"
@@ -59,6 +62,12 @@ EXPORT_SYMBOL(pgtable_l4_enabled);
 EXPORT_SYMBOL(pgtable_l5_enabled);
 #endif
 
+#ifdef CONFIG_SPARSEMEM
+#define RISCV_MEMSTART_ALIGN	(1UL << SECTION_SIZE_BITS)
+#else
+#define RISCV_MEMSTART_ALIGN	PMD_SIZE
+#endif
+
 phys_addr_t phys_ram_base __ro_after_init;
 EXPORT_SYMBOL(phys_ram_base);
 
@@ -239,9 +248,13 @@ static void __init setup_bootmem(void)
 	/*
 	 * Make sure we align the start of the memory on a PMD boundary so that
 	 * at worst, we map the linear mapping with PMD mappings.
+	 *
+	 * Also, make sure we align the start of the memory on a SECTION boundary
+	 * when CONFIG_SPARSEMEM_VMEMMAP is enabled to ensure the correctness of
+	 * pfn_to_page().
 	 */
 	if (!IS_ENABLED(CONFIG_XIP_KERNEL))
-		phys_ram_base = memblock_start_of_DRAM() & PMD_MASK;
+		phys_ram_base = round_down(memblock_start_of_DRAM(), RISCV_MEMSTART_ALIGN);
 
 	/*
 	 * In 64-bit, any use of __va/__pa before this point is wrong as we
-- 
2.20.1