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

Mason
2016-05-13T08:44:52Z

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.