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Date:   Tue, 16 Feb 2021 08:42:26 +0530
From:   Anshuman Khandual <>
To:     Pavel Tatashin <>,
        Ard Biesheuvel <>
Cc:     Tyler Hicks <>,
        James Morris <>,
        Catalin Marinas <>,
        Will Deacon <>,
        Andrew Morton <>,
        Mike Rapoport <>,
        Logan Gunthorpe <>,
        Linux ARM <>,
        Linux Kernel Mailing List <>
Subject: Re: [PATCH v2 1/1] arm64: mm: correct the inside linear map
 boundaries during hotplug check

On 2/16/21 1:21 AM, Pavel Tatashin wrote:
> On Mon, Feb 15, 2021 at 2:34 PM Ard Biesheuvel <> wrote:
>> On Mon, 15 Feb 2021 at 20:30, Pavel Tatashin <> wrote:
>>>> Can't we simply use signed arithmetic here? This expression works fine
>>>> if the quantities are all interpreted as s64 instead of u64
>>> I was thinking about that, but I do not like the idea of using sign
>>> arithmetics for physical addresses. Also, I am worried that someone in
>>> the future will unknowingly change it to unsigns or to phys_addr_t. It
>>> is safer to have start explicitly set to 0 in case of wrap.
>> memstart_addr is already a s64 for this exact reason.
> memstart_addr is basically an offset and it must be negative. For
> example, this would not work if it was not signed:
> #define vmemmap ((struct page *)VMEMMAP_START - (memstart_addr >> PAGE_SHIFT))
> However, on powerpc it is phys_addr_t type.
>> Btw, the KASLR check is incorrect: memstart_addr could also be
>> negative when running the 52-bit VA kernel on hardware that is only
>> 48-bit VA capable.
> Good point!
> if (IS_ENABLED(CONFIG_ARM64_VA_BITS_52) && (vabits_actual != 52))
>     memstart_addr -= _PAGE_OFFSET(48) - _PAGE_OFFSET(52);
> So, I will remove IS_ENABLED(CONFIG_RANDOMIZE_BASE) again.
> I am OK to change start_linear_pa, end_linear_pa to signed, but IMO
> what I have now is actually safer to make sure that does not break
> again in the future.
An explicit check for the flip over and providing two different start
addresses points would be required in order to use the new framework.

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