Hi Alexandre,
I have a few questions about this patch. Sorry to be dense here ...
On Wed, 24 Jul 2019, Alexandre Ghiti wrote:
> In order to avoid wasting user address space by using bottom-up mmap
> allocation scheme, prefer top-down scheme when possible.
>
> Before:
> root@qemuriscv64:~# cat /proc/self/maps
> 00010000-00016000 r-xp 00000000 fe:00 6389 /bin/cat.coreutils
> 00016000-00017000 r--p 00005000 fe:00 6389 /bin/cat.coreutils
> 00017000-00018000 rw-p 00006000 fe:00 6389 /bin/cat.coreutils
> 00018000-00039000 rw-p 00000000 00:00 0 [heap]
> 1555556000-155556d000 r-xp 00000000 fe:00 7193 /lib/ld-2.28.so
> 155556d000-155556e000 r--p 00016000 fe:00 7193 /lib/ld-2.28.so
> 155556e000-155556f000 rw-p 00017000 fe:00 7193 /lib/ld-2.28.so
> 155556f000-1555570000 rw-p 00000000 00:00 0
> 1555570000-1555572000 r-xp 00000000 00:00 0 [vdso]
> 1555574000-1555576000 rw-p 00000000 00:00 0
> 1555576000-1555674000 r-xp 00000000 fe:00 7187 /lib/libc-2.28.so
> 1555674000-1555678000 r--p 000fd000 fe:00 7187 /lib/libc-2.28.so
> 1555678000-155567a000 rw-p 00101000 fe:00 7187 /lib/libc-2.28.so
> 155567a000-15556a0000 rw-p 00000000 00:00 0
> 3fffb90000-3fffbb1000 rw-p 00000000 00:00 0 [stack]
>
> After:
> root@qemuriscv64:~# cat /proc/self/maps
> 00010000-00016000 r-xp 00000000 fe:00 6389 /bin/cat.coreutils
> 00016000-00017000 r--p 00005000 fe:00 6389 /bin/cat.coreutils
> 00017000-00018000 rw-p 00006000 fe:00 6389 /bin/cat.coreutils
> 2de81000-2dea2000 rw-p 00000000 00:00 0 [heap]
> 3ff7eb6000-3ff7ed8000 rw-p 00000000 00:00 0
> 3ff7ed8000-3ff7fd6000 r-xp 00000000 fe:00 7187 /lib/libc-2.28.so
> 3ff7fd6000-3ff7fda000 r--p 000fd000 fe:00 7187 /lib/libc-2.28.so
> 3ff7fda000-3ff7fdc000 rw-p 00101000 fe:00 7187 /lib/libc-2.28.so
> 3ff7fdc000-3ff7fe2000 rw-p 00000000 00:00 0
> 3ff7fe4000-3ff7fe6000 r-xp 00000000 00:00 0 [vdso]
> 3ff7fe6000-3ff7ffd000 r-xp 00000000 fe:00 7193 /lib/ld-2.28.so
> 3ff7ffd000-3ff7ffe000 r--p 00016000 fe:00 7193 /lib/ld-2.28.so
> 3ff7ffe000-3ff7fff000 rw-p 00017000 fe:00 7193 /lib/ld-2.28.so
> 3ff7fff000-3ff8000000 rw-p 00000000 00:00 0
> 3fff888000-3fff8a9000 rw-p 00000000 00:00 0 [stack]
>
> Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>
> Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
> arch/riscv/Kconfig | 11 +++++++++++
> 1 file changed, 11 insertions(+)
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 59a4727ecd6c..6a63973873fd 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -54,6 +54,17 @@ config RISCV
> select EDAC_SUPPORT
> select ARCH_HAS_GIGANTIC_PAGE
> select ARCH_WANT_HUGE_PMD_SHARE if 64BIT
> + select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT if MMU
> + select HAVE_ARCH_MMAP_RND_BITS
> +
> +config ARCH_MMAP_RND_BITS_MIN
> + default 18
Could you help me understand the rationale behind this constant?
> +
> +# max bits determined by the following formula:
> +# VA_BITS - PAGE_SHIFT - 3
I realize that these lines are probably copied from arch/arm64/Kconfig.
But the rationale behind the "- 3" is not immediately obvious. This
apparently originates from commit 8f0d3aa9de57 ("arm64: mm: support
ARCH_MMAP_RND_BITS"). Can you provide any additional context here?
> +config ARCH_MMAP_RND_BITS_MAX
> + default 33 if 64BIT # SV48 based
The rationale here is clear for Sv48, per the above formula:
(48 - 12 - 3) = 33
> + default 18
However, here it is less clear to me. For Sv39, shouldn't this be
(39 - 12 - 3) = 24
? And what about Sv32?
- Paul
