[PATCH] arm64: Support ARM64_VA_BITS=52 when setting ARCH_MMAP_RND_BITS_MAX

[Anshuman Khandual]

On 4/4/25 22:16, Kornel Dulęba wrote:
> On Thu, Apr 3, 2025 at 11:46 PM Anshuman Khandual
> <anshuman.khandual at arm.com> wrote:
>>
>>
>>
>> On 4/4/25 00:06, Kornel Dulęba wrote:
>>> When the 52-bit virtual addressing was enabled the select like
>>> ARCH_MMAP_RND_BITS_MAX logic was never updated to account for it.
>>> Because of that the rnd max bits would be set to the default value of
>>> 18 when ARM64_VA_BITS=52.
>>> Fix this by setting ARCH_MMAP_RND_BITS_MAX to the same value that would
>>> be used if 48-bit addressing was used. That's because the 52-bit
>>> addressing is used only if the caller provides a hint to mmap, with a
>>> fallback to 48-bit addressing.
>>
>> Why should ARCH_MMAP_RND_BITS_MAX value be same for both 48 bits and 52
>> bits VA in case the user does request for 52 bit VA via mmap() hint and
>> the HW supports it ?
> 
> Two reasons really.
> 1. The whole behavior is controlled through a global knob -
> /proc/sys/vm/mmap_rnd_bits. ARCH_MMAP_RND_BITS_MAX is used as an upper
> bound for the value that can be set to that knob.
> So we have a single setting for all processes. Some might want 52 bit
> addressing, others will stick with 48.
> 2. Quoting the documentation for this knob:
> 
> """
> mmap_rnd_bits
> This value can be used to select the number of bits to use to
> determine the random offset to the base address of vma regions
> resulting from mmap allocations on architectures which support tuning
> address space randomization. This value will be bounded by the
> architecture’s minimum and maximum supported values.
> """
> 
> I suppose that it's legal for some calls to mmap from the same process
> to request a 52 bit VA, while other calls will want only 48 bits.
> Because of that the random offset can't be larger than what would work
> for the 48 bit case.

Agreed but should not this rationale also be added in the commit
message as well ?

> 
>>
>>>
>>> Fixes: b6d00d47e81a ("arm64: mm: Introduce 52-bit Kernel VAs")

Correct commit to be attributed for this fix.

>>> Signed-off-by: Kornel Dulęba <korneld at google.com>
>>> ---
>>>  arch/arm64/Kconfig | 6 +++---
>>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>>> index 748c34dc953c..38e0bac567f5 100644
>>> --- a/arch/arm64/Kconfig
>>> +++ b/arch/arm64/Kconfig
>>> @@ -332,9 +332,9 @@ config ARCH_MMAP_RND_BITS_MAX
>>>       default 24 if ARM64_VA_BITS=39
>>>       default 27 if ARM64_VA_BITS=42
>>>       default 30 if ARM64_VA_BITS=47
>>> -     default 29 if ARM64_VA_BITS=48 && ARM64_64K_PAGES
>>> -     default 31 if ARM64_VA_BITS=48 && ARM64_16K_PAGES
>>> -     default 33 if ARM64_VA_BITS=48
>>> +     default 29 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52) && ARM64_64K_PAGES
>>> +     default 31 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52) && ARM64_16K_PAGES
>>> +     default 33 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52)
>>>       default 14 if ARM64_64K_PAGES
>>>       default 16 if ARM64_16K_PAGES
>>>       default 18

Otherwise LGTM.

Reviewed-by: Anshuman Khandual <anshuman.khandual at arm.com>