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Date: Tue, 1 Apr 2014 19:01:58 +0200
From: Szabolcs Nagy <>
Subject: Re: Proposed approach for malloc to deal with failing brk

* Vasily Kulikov <> [2014-04-01 20:40:57 +0400]:
> On Sun, Mar 30, 2014 at 20:41 -0400, Rich Felker wrote:
> > We want brk. This is not because "brk is faster than mmap", but
> > because it takes a lot of work to replicate what brk does using mmap,
> > and there's no hope of making a complex dance of multiple syscalls
> > equally efficient. My best idea for emulating brk was to mmap a huge
> > PROT_NONE region and gradually mprotect it to PROT_READ|PROT_WRITE,
> What problem do you try to solve via PROT_NONE -> PROT_WRITE?  Why not

writable page is commit charge and that matters with a huge mmap
on systems with no overcommit

> simply instantly mmap it as PROT_WRITE?  Linux will not allocate physical pages
> until the first access, so you don't lose physical memory when it is not
> actually used.

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