Re: [PATCH] mm: export mmu notifier invalidates

[Robin Holt <holt@sgi.com>]

On Thu, Feb 14, 2013 at 01:52:34PM -0800, Andrew Morton wrote:
> On Thu, 14 Feb 2013 15:35:12 -0600
> Robin Holt <holt@sgi.com> wrote:
> 
> > I am open to suggestions.  Can you suggest existing kernel functionality
> > that allows one task to map another virtual address space into their
> > va space to allow userland-to-userland copies without system calls?
> > If there is functionality that has been introduced in the last couple
> > years, I could very well have missed it as I have been fairly heads-down
> > on other things for some time.
> 
> That's conceptually very similar to mm/process_vm_access.c. 
> process_vm_readv/writev do kernel-based copying rather than a direct
> mmap.

I will go look at those now.  I am not familiar with them as they went
in during my "dark period" where I was working on system controller
functionality and not paying attention to kernel activity.

> 
> > > To what extent is all this specific to SGI hardware characteristics?
> > 
> > SGI's hardware allows two things, a vastly larger virtual address space
> > and the ability to access memory in other system images on the same numa
> > fabric which are beyond the processsors physical addressing capabilities.
> > 
> > I am fairly sure Cray has taken an older version of XPMEM and stripped
> > out a bunch of SGI specific bits and implemented it on their hardware.
> > 
> > > > The above, of course, is an oversimplification, but should give you and
> > > > idea of the big picture design goals.
> > > >
> > > > Does any of this make sense?  Do you see areas where you think we should
> > > > extend regular mm functionality to include these functions?
> > > > 
> > > > How would you like me to proceed?
> > > 
> > > I'm obviously on first base here, but overall approach:
> > > 
> > > - Is the top-level feature useful to general Linux users?  Perhaps
> > >   after suitable generalisations (aka dumbing down :))
> > 
> > I am not sure how useful it is.  I know IBM has tried in the past to
> > get a similar feature introduced.  I believe they settled on a ptrace
> > extension to do direct user-to-user copies from within the kernel.
> 
> process_vm_readv/writev is from Christopher Yeoh@IBM.
> 
> > > - Even if the answer to that is "no", should we maintain the feature
> > >   in-tree rather than out-of-tree?
> > 
> > Not sure on the second one, but I believe Linus' objection is security and
> > I can certainly understand that.  Right now, SGI's xpmem implementation
> > enforces that all jobs in the task need to have the same UID.  There is
> > no exception for root or and administrator.
> 
> I'd have thought that the security processing of a direct map would be
> identical to those in process_vm_readv/writev?
> 
> If we were to add a general map-this-into-that facility which is
> available to and runs adequately on our typical machines, I assume your
> systems would need some SGI-specific augmentation?

Yes, for the extended virtual and physical address space and for the
weird page sizes.

Thanks,
Robin

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