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

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Message-ID: <871pypq942.fsf@all.your.base.are.belong.to.us>
Date: Tue, 03 Dec 2024 12:49:49 +0100
From: Björn Töpel <bjorn@...nel.org>
To: Xu Lu <luxu.kernel@...edance.com>, 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: Re: [PATCH] riscv: mm: Fix alignment of phys_ram_base

Xu Lu <luxu.kernel@...edance.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@...edance.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