Re: [PATCH -mm] Revoke core code: fix nommu arch compiling error bug

David Howells
2007-03-26T12:23:25Z

Pekka J Enberg <penberg@cs.helsinki.fi> wrote:

> > I don't know, what does it do? Remember, once a NOMMU process thinks it
> > has the right to access a mapping, there's no way of stopping it doing so
> > short of killing the process.
>
> revoke_mapping() is mostly same as munmap(2) except that it preserves the
> vma but makes it VM_REVOKED. This means that if the process tries to
> access the region it will SIGBUS and if it tries to remap the range it
> will get EINVAL.

Yeah, that's not enforceable in NOMMU-mode situations. I presume it differs
from munmap() also in that it can effectively be forced by one process upon
another.

In MMU-mode, how does this work with private mappings that have some private
copies of the pages that make up the mapping? Are those still available to a
process that is using them? Are they revoked when swapped out? Or are they
forcibly evicted?

> What we're trying to do here is, we want to make sure no other tasks can
> access the inode once it has been revoked.

Okay.

> So there's no way to raise SIGBUS if the range is being accessed. The
> alternatives are:
>
> - No support for revoke(2) on NOMMU.

That's a bit over the top, I think. It sounds like revoke() is perfectly fine
- as long as there aren't any mappings on the target inode (or at least shared
mappings - dunno about private mappings).

> - If there are shared mappings, always return -ENOENT for revoke(2).

That sounds feasible. How about -ETXTBSY instead?

> - If there are shared mappings, immediately raise SIGBUS for those
> processes that are accessing it.

Hmmm... maybe. That sounds a bit antisocial though, but is also workable.
Does the SIGBUS raised have its own si_code, btw? Perhaps BUS_REVOKED?

> Making the shared mappings private is not an option because there's no way
> for the process to know that it's mapping is being pulled under it which
> will result in bugs. Hmm.

Agreed.

David