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

[Sebastian Frias <sf84@laposte.net>]

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

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