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

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Date:	Mon, 23 May 2016 15:11:44 +0200
From:	Sebastian Frias <sf84@...oste.net>
To:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Cc:	Michal Hocko <mhocko@...nel.org>, Mason <slash.tmp@...e.fr>,
	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

Hi Alan,

On 05/13/2016 05:41 PM, One Thousand Gnomes wrote:
>> My understanding is that there was a time when there was no overcommit at all.
>> If that's the case, understanding why overcommit was introduced would be helpful.
> 
> Linux always had overcommit.
> 
> The origin of overcommit is virtual memory for the most part. In a
> classic swapping system without VM the meaning of brk() and thus malloc()
> is that it allocates memory (or swap). Likewise this is true of fork()
> and stack extension.
> 
> In a virtual memory system these allocate _address space_. It does not
> become populated except by page faulting, copy on write and the like. It
> turns out that for most use cases on a virtual memory system we get huge
> amounts of page sharing or untouched space.
> 
> Historically Linux did guess based overcommit and I added no overcommit
> support way back when, along with 'anything is allowed' support for
> certain HPC use cases.
> 
> The beancounter patches combined with this made the entire setup
> completely robust but the beancounters never hit upstream although years
> later they became part of the basis of the cgroups.
> 
> You can sort of set a current Linux up for definitely no overcommit using
> cgroups and no overcommit settings. It works for most stuff although last
> I checked most graphics drivers were terminally broken (and not just to
> no overcommit but to the point you can remote DoS Linux boxes with a
> suitably constructed web page and chrome browser)
> 
> Alan
> 

Thanks for your comment, it certainly provides more clues and provided some history about the "overcommit" setting.
I will see if we can do what we want with cgroups.

Best regards,

Sebastian