linux-kernel - [PATCH] riscv: mm: Fix alignment of phys_ram

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Message-Id: <20241202101601.48284-1-luxu.kernel@bytedance.com>
Date: Mon,  2 Dec 2024 18:16:01 +0800
From: Xu Lu <luxu.kernel@...edance.com>
To: paul.walmsley@...ive.com,
	palmer@...belt.com,
	aou@...s.berkeley.edu
Cc: lihangjing@...edance.com,
	xieyongji@...edance.com,
	linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	Xu Lu <luxu.kernel@...edance.com>
Subject: [PATCH] riscv: mm: Fix alignment of phys_ram_base

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_unavailage_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.

Signed-off-by: Xu Lu <luxu.kernel@...edance.com>
---
 arch/riscv/mm/init.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index 0e8c20adcd98..9866de267b74 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -59,6 +59,8 @@ EXPORT_SYMBOL(pgtable_l4_enabled);
 EXPORT_SYMBOL(pgtable_l5_enabled);
 #endif

+#define RISCV_MEMSTART_ALIGN	(1UL << SECTION_SIZE_BITS)
+
 phys_addr_t phys_ram_base __ro_after_init;
 EXPORT_SYMBOL(phys_ram_base);

@@ -241,7 +243,8 @@ static void __init setup_bootmem(void)
 	 * at worst, we map the linear mapping with PMD mappings.
 	 */
 	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