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Date: Thu, 25 Jul 2019 17:20:50 -0700 (PDT) From: Paul Walmsley <paul.walmsley@...ive.com> To: Alexandre Ghiti <alex@...ti.fr> cc: Andrew Morton <akpm@...ux-foundation.org>, Albert Ou <aou@...s.berkeley.edu>, Daniel Cashman <dcashman@...gle.com>, Kees Cook <keescook@...omium.org>, Catalin Marinas <catalin.marinas@....com>, Palmer Dabbelt <palmer@...ive.com>, Will Deacon <will.deacon@....com>, Russell King <linux@...linux.org.uk>, Ralf Baechle <ralf@...ux-mips.org>, linux-kernel@...r.kernel.org, linux-mm@...ck.org, Paul Burton <paul.burton@...s.com>, Alexander Viro <viro@...iv.linux.org.uk>, James Hogan <jhogan@...nel.org>, linux-fsdevel@...r.kernel.org, linux-riscv@...ts.infradead.org, linux-mips@...r.kernel.org, Christoph Hellwig <hch@....de>, linux-arm-kernel@...ts.infradead.org, Luis Chamberlain <mcgrof@...nel.org> Subject: Re: [PATCH REBASE v4 14/14] riscv: Make mmap allocation top-down by default 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@...uriscv64:~# 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@...uriscv64:~# 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@...ti.fr> > Reviewed-by: Christoph Hellwig <hch@....de> > Reviewed-by: Kees Cook <keescook@...omium.org> > --- > 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
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