linux-kernel - Re: mmap, the language go, problems with the linux kernel

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Date:	Wed, 16 Feb 2011 21:51:32 +0100
From:	Florian Weimer <fw@...eb.enyo.de>
To:	Ted Ts'o <tytso@....edu>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	martin capitanio <m@...itanio.org>,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org
Subject: Re: mmap, the language go, problems with the linux kernel

* Ted Ts'o:

>> The odd thing is that prot==0 does *not* count against the
>> vm.overcommit_memory=2 limit, only against ulimit -v.  The limit is
>> only enforced for the parts on which mprotect is called.  I think this
>> should really be part of the public API (I'm not sure if it is right
>> now, it could well be an accident), to avoid the problems you
>> describe.
>
> The overcommit_memory logic does not include any pages which are
> mapped read-only.

A colleague tells me that according to his tests, this depends on the
history of the page, as expected.

> Technically that's not quite enough --- in theory you could have a
> debugging attach to every single read-only text page and set
> breakpoints on every single page.

Those cases do not matter because setting a breakpoint can fail with
ENOMEM.  You only have to take into account possibly future operations
which cannot fail with ENOMEM.
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