On Apr 16, 2024, at 4:36 PM, Nam Cao <namcao@xxxxxxxxxxxxx> wrote:
>
> On 2024-04-16 Mike Rapoport wrote:
>> On Tue, Apr 16, 2024 at 06:00:29PM +0100, Matthew Wilcox wrote:
>>> On Tue, Apr 16, 2024 at 07:31:54PM +0300, Mike Rapoport wrote:
>>>>> - if (!IS_ENABLED(CONFIG_64BIT)) {
>>>>> - max_mapped_addr = __pa(~(ulong)0);
>>>>> - if (max_mapped_addr == (phys_ram_end - 1))
>>>>> - memblock_set_current_limit(max_mapped_addr - 4096);
>>>>> - }
>>>>> + memblock_reserve(__pa(-PAGE_SIZE), PAGE_SIZE);
>>>>
>>>> Ack.
>>>
>>> Can this go to generic code instead of letting architecture maintainers
>>> fall over it?
>>
>> Yes, it's just have to happen before setup_arch() where most architectures
>> enable memblock allocations.
>
> This also works, the reported problem disappears.
>
> However, I am confused about one thing: doesn't this make one page of
> physical memory inaccessible?
>
> Is it better to solve this by setting max_low_pfn instead? Then at
> least the page is still accessible as high memory.
Is that one page of memory really worthwhile to preserve? Better to
have a simple solution that works, maybe even mapping that page
read-only so that any code which tries to dereference an ERR_PTR
address immediately gets a fault?
Cheers, Andreas
>
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index fa34cf55037b..6e3130cae675 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -197,7 +197,6 @@ early_param("mem", early_mem);
> static void __init setup_bootmem(void)
> {
> phys_addr_t vmlinux_end = __pa_symbol(&_end);
> - phys_addr_t max_mapped_addr;
> phys_addr_t phys_ram_end, vmlinux_start;
>
> if (IS_ENABLED(CONFIG_XIP_KERNEL))
> @@ -235,23 +234,9 @@ static void __init setup_bootmem(void)
> if (IS_ENABLED(CONFIG_64BIT))
> kernel_map.va_pa_offset = PAGE_OFFSET - phys_ram_base;
>
> - /*
> - * memblock allocator is not aware of the fact that last 4K bytes of
> - * the addressable memory can not be mapped because of IS_ERR_VALUE
> - * macro. Make sure that last 4k bytes are not usable by memblock
> - * if end of dram is equal to maximum addressable memory. For 64-bit
> - * kernel, this problem can't happen here as the end of the virtual
> - * address space is occupied by the kernel mapping then this check must
> - * be done as soon as the kernel mapping base address is determined.
> - */
> - if (!IS_ENABLED(CONFIG_64BIT)) {
> - max_mapped_addr = __pa(~(ulong)0);
> - if (max_mapped_addr == (phys_ram_end - 1))
> - memblock_set_current_limit(max_mapped_addr - 4096);
> - }
> -
> min_low_pfn = PFN_UP(phys_ram_base);
> - max_low_pfn = max_pfn = PFN_DOWN(phys_ram_end);
> + max_pfn = PFN_DOWN(phys_ram_end);
> + max_low_pfn = min(max_pfn, PFN_DOWN(__pa(-PAGE_SIZE)));
> high_memory = (void *)(__va(PFN_PHYS(max_low_pfn)));
>
> dma32_phys_limit = min(4UL * SZ_1G, (unsigned long)PFN_PHYS(max_low_pfn));
Cheers, Andreas
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