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

[Kornel Dulęba]

On Thu, Apr 17, 2025 at 7:15 AM Anshuman Khandual
<anshuman.khandual at arm.com> wrote:
>
>
>
> 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 ?

Sure, I will update this in v2.
>
>
> >
> >>
> >>>
> >>> Fixes: b6d00d47e81a ("arm64: mm: Introduce 52-bit Kernel VAs")
>
> Correct commit to be attributed for this fix.

What commit would you like me to point at? I selected b6d00d47e81a,
because it introduced "ARM64_VA_BITS=52".
Looking at git blame, ARCH_MMAP_RND_BITS_MAX logic was introduced back
in 2016 in 8f0d3aa9de57 ("arm64: mm: support ARCH_MMAP_RND_BITS"),
which is before "ARM64_VA_BITS=52" was a thing.
I suppose that there's also 3cb7e662a930 ("arm64: Kconfig: Fix
indentation and add comments"), but that's just some whitespace
adjustments.

>
>
> >>> 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>