linux-kernel - Re: [PATCH] mm: add config option to select the initial overcommit mode

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Date:	Fri, 13 May 2016 10:44:30 +0200
From:	Mason <slash.tmp@...e.fr>
To:	Michal Hocko <mhocko@...nel.org>,
	Sebastian Frias <sf84@...oste.net>
Cc:	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: add config option to select the initial overcommit
 mode

On 13/05/2016 10:04, Michal Hocko wrote:

> On Tue 10-05-16 13:56:30, Sebastian Frias wrote:
> [...]
>> NOTE: I understand that the overcommit mode can be changed dynamically thru
>> sysctl, but on embedded systems, where we know in advance that overcommit
>> will be disabled, there's no reason to postpone such setting.
> 
> To be honest I am not particularly happy about yet another config
> option. At least not without a strong reason (the one above doesn't
> sound that way). The config space is really large already.
> So why a later initialization matters at all? Early userspace shouldn't
> consume too much address space to blow up later, no?

One thing I'm not quite clear on is: why was the default set
to over-commit on?

I suppose the biggest use-case is when a "large" process forks
only to exec microseconds later into a "small" process, it would
be silly to refuse that fork. But isn't that what the COW
optimization addresses, without the need for over-commit?

Another issue with overcommit=on is that some programmers seem
to take for granted that "allocations will never fail" and so
neglect to handle malloc == NULL conditions gracefully.

I tried to run LTP with overcommit off, and I vaguely recall that
I had more failures than with overcommit on. (Perhaps only those
tests that tickle the dreaded OOM assassin.)

Regards.