Re: [PATCH REBASE v4 14/14] riscv: Make mmap allocation top-down by default
From: Paul Walmsley <hidden>
Date: 2019-07-26 00:20:58
Also in:
linux-fsdevel, linux-mips, linux-mm, linux-riscv, lkml
Hi Alexandre, I have a few questions about this patch. Sorry to be dense here ... On Wed, 24 Jul 2019, Alexandre Ghiti wrote:
quoted hunk ↗ jump to hunk
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 <redacted> --- 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 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel