[PATCH] riscv: mm: Fix alignment of phys_ram_base
Björn Töpel
bjorn at kernel.org
Tue Dec 3 03:49:49 PST 2024
Xu Lu <luxu.kernel at bytedance.com> writes:
> 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.
Nice catch! I managed to reproduce this on a hacked qemu virt machine.
> 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
"init_unavailable_range()" spelling for greppability.
> 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 at bytedance.com>
Please add a fixes tag.
> ---
> 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);
No need to wrap this line. Also, is the RISCV_MEMSTART_ALIGN define
really needed?
The kernel test robot had some build issues as well!
Björn
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