Re: [PATCH v5 14/14] riscv: Make mmap allocation top-down by default

[Alex Ghiti <alex@ghiti.fr>]

On 7/30/19 1:51 AM, 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@ghiti.fr>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
> ---
>   arch/riscv/Kconfig | 13 +++++++++++++
>   1 file changed, 13 insertions(+)
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 8ef64fe2c2b3..8d0d8af1a744 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -54,6 +54,19 @@ 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 if 64BIT
> +	default 8
> +
> +# max bits determined by the following formula:
> +#  VA_BITS - PAGE_SHIFT - 3
> +config ARCH_MMAP_RND_BITS_MAX
> +	default 33 if RISCV_VM_SV48
> +	default 24 if RISCV_VM_SV39
> +	default 17 if RISCV_VM_SV32
>   
>   config MMU
>   	def_bool y


Hi Andrew,

I have just seen you took this series into mmotm but without Paul's 
patch ("riscv: kbuild: add virtual memory system selection") on which 
this commit relies, I'm not sure it could
compile without it as there is no default for ARCH_MMAP_RND_BITS_MAX.

Thanks,

Alex