Re: [PATCH] recognize MAP_LOCKED in mmap() call

[Andrew Morton <akpm@digeo.com>]

Mark_H_Johnson@raytheon.com wrote:
> 
> Andrew Morton wrote:
> >(SuS really only anticipates that mmap needs to look at prior mlocks
> >in force against the address range.  It also says
> >
> >     Process memory locking does apply to shared memory regions,
> >
> >and we don't do that either.  I think we should; can't see why SuS
> >requires this.)
> 
> Let me make sure I read what you said correctly. Does this mean that Linux
> 2.4 (or 2.5) kernels do not lock shared memory regions if a process uses
> mlockall?

Linux does lock these regions.  SuS seems to imply that we shouldn't.
But we should.

> If not, that is *really bad* for our real time applications. We don't want
> to take a page fault while running some 80hz task, just because some
> non-real time application tried to use what little physical memory we allow
> for the kernel and all other applications.
> 
> I asked a related question about a week ago on linux-mm and didn't get a
> response. Basically, I was concerned that top did not show RSS == Size when
> mlockall(MCL_CURRENT|MCL_FUTURE) was called. Could this explain the
> difference or is there something else that I'm missing here?
> 

That mlockall should have faulted everything in.  It could be an
accounting bug, or it could be a bug.  That's not an aspect which
gets tested a lot.  I'll take a look.