Re: remap_file_pages() use

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

Jeff Smith wrote:
> >> Mirrored mapping is absolutely required by several
> >> independent proprietary platforms I'm aware of, and remap_file_pages()
> >> has historically been the only sane way to accomplish this. (i.e.,
> >> shm_open(), mmap(NULL, 2^(n+1) pages), remap_file_pages() on 2nd
> >> half).
> >
> > Em.. What's wrong with shm_open() + two mmap()s to cover both halfs?
> >
> > fd = shm_open();
> > addr1 = mmap(NULL, 2*SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
> > addr2 = mmap(addr1 + SIZE, PROT_READ|PROT_WRITE, MAP_SHARED | MAP_FIXED, fd, 0);
> >
> > Is there a reason why it doens't work?
> 
> Your addr2 mmap() call is a bit incorrect semantically and
> syntactically (you skipped the length arg).

My bad.

> The addr2 request will fail because mmap() does not implicitly munmap()
> occupied virtual address space.

Please, consider reading man page for mmap(2). MAP_FIXED in particular.

> Even if you did that, the following still has a race
> condition between the addr2 request and another thread grabbing the
> same virtual space, which nothing short of a lock on all threads'
> mmap()-ing logic can protect:
> 
> addr1 = mmap(NULL, 2*SIZE, PROT_READ, MAP_SHARED, fd, 0);
> munmap(addr1 + SIZE, SIZE);
> /* race on virtual address space here, but n/a for remap_file_pages() ... */
> addr2 = mmap(addr1, SIZE, PROT_READ, MAP_SHARED | MAP_FIXED, fd, 0);

No. MAP_FIXED will do the job: it does munmap() + mmap() atomically from
userspace POV.

> >> but failing that, a reservation API would need
> >> to be created (possibly a MAP_RESERVE flag) that would set aside a
> >> region that could only be subsequently mapped via explicit
> >> address-requesting mmap() calls.
> >
> > I don't get this part.
> 
> I'm proposing that a call along the lines of mmap(NULL, len, prot,
> MAP_RESERVED | ..., fd, offset) could return a virtual address block
> that is -not- actually mapped but -is- protected from other mmap()
> calls not explicitly requesting the space via their addr parameters.
> Unfortunately, you'd also need to define separate semantics to
> un-reserving not-mapped space, etc.

You're inventing a wheel. All you need is there for ages. And in portable
way.

-- 
 Kirill A. Shutemov