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Re: When should ralloc.c be used?

From: Stefan Monnier
Subject: Re: When should ralloc.c be used?
Date: Fri, 28 Oct 2016 08:11:17 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.1.50 (gnu/linux)

> If reserving a range of addresses doesn't necessarily mean they will
> be later available for committing, then what is the purpose of
> reserving them in the first place?  What good does it do?

My guess is that you can later use that address space to mmap files
in there.  Equivalently, you could increase the swap space between the
time you PROT_NONE and the time you switch to PROT_RW.

PROT_NONE is useful in a situation such as ours: you want to mmap
a hundred buffers, and make sure you can grow any of them without
knowing beforehand which one will grow.

But most likely, whether it's useful or not to be able to reserve 80TB
of address space even if you'd never be able to PROT_RW later was not
really relevant to the design of the API.


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