Re: [PATCH] MM: Support more pagesizes for MAP_HUGETLB/SHM_HUGETLBv6

From: Andrew Morton
Date: Mon Oct 22 2012 - 17:39:31 EST

On Mon, 22 Oct 2012 15:27:33 +0200
Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:

> BTW seriously MAP_UNINITIALIZED? Who came up with that?
> MAP_COMPLETELY_INSECURE or MAP_INSANE would have been more appropiate.

heh. It's a NOMMU-only thing.

bool "Allow mmapped anonymous memory to be uninitialized"
depends on EXPERT && !MMU
default n
Normally, and according to the Linux spec, anonymous memory obtained
from mmap() has it's contents cleared before it is passed to
userspace. Enabling this config option allows you to request that
mmap() skip that if it is given an MAP_UNINITIALIZED flag, thus
providing a huge performance boost. If this option is not enabled,
then the flag will be ignored.

This is taken advantage of by uClibc's malloc(), and also by
ELF-FDPIC binfmt's brk and stack allocator.

Because of the obvious security issues, this option should only be
enabled on embedded devices where you control what is run in
userspace. Since that isn't generally a problem on no-MMU systems,
it is normally safe to say Y here.

See Documentation/nommu-mmap.txt for more information.

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