Re: [RFC] OVERCOMMIT_ALWAYS extension

[Hugh Dickins <hugh@veritas.com>]

On Mon, 17 Oct 2005, Badari Pulavarty wrote:
> 
> I have been looking at possible ways to extend OVERCOMMIT_ALWAYS
> to avoid its abuse.
> 
> Few of the applications (database) would like to overcommit
> memory (by creating shared memory segments more than RAM+swap),
> but use only portion of it at any given time and get rid
> of portions of them through madvise(DONTNEED), when needed. 
> They want this, especially to handle hotplug memory situations 
> (where apps may not have clear idea on how much memory they have 
> in the system at the time of shared memory create). Currently, 
> they are using OVERCOMMIT_ALWAYS system wide to do this - but 
> they are affecting every other application on the system.
> 
> I am wondering, if there is a better way to do this. Simple solution
> would be to add IPC_OVERCOMMIT flag or add CAP_SYS_ADMIN to
> do the overcommit. This way only specific applications, requesting
> this would be able to overcommit. I am worried about, the over
> all affects it has on the system. But again, this can't be worse
> than system wide  OVERCOMMIT_ALWAYS. Isn't it ?

mmap has MAP_NORESERVE, without CAP_SYS_ADMIN or other restriction,
which exempts that mmap from security_vm_enough_memory checking -
unless current setting is OVERCOMMIT_NEVER, in which case
MAP_NORESERVE is ignored.

So if you're content to move to the OVERCOMMIT_GUESS world, I
don't think you could be blamed for adding an IPC_NORESERVE which
behaves in the same way, without CAP_SYS_ADMIN restriction.

But if you want to move to OVERCOMMIT_NEVER, yet have a flag which
says overcommit now, you'll get into a tussle with NEVER-adherents.

Hugh

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