Re: remap_file_pages() use

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

Jeff Smith wrote:
> On Mon, May 19, 2014 at 9:38 AM, Christoph Hellwig <hch@infradead.org> wrote:
> > On Mon, May 19, 2014 at 05:35:40PM +0300, Kirill A. Shutemov wrote:
> >> >From functional POV, emulation *should* be identical to original
> >> remap_file_pages(), but slower. It would be nice, if you test it early.
> >>
> >> It's not clear yet how long emulation will be there.
> >
> > Stop right there.  We found out about two real life users of
> > remap_file_pages() already, without even committing the patches to warn
> > about using it to any tree.
> >
> > I think at this point the whole idea of removing the API should be dead
> > on the floor, as we do not needlessly break userspace programs.
> >
> > If we can get rid of the ugly guts and provide a good enough emulation
> > that the user won't cry I'd love to get rid of this cruft, but even
> > that doesn't look certain yet.
> 
> Sorry for being late to the party, but I just noticed this proposal
> via the LWN summary byline.
> 
> I wanted to comment that Kenny's use case is (I believe) quite
> widespread. I've used the technique since ~2008, and I've come across
> other people in subsequent jobs who independently developed the same
> technique. 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?

> It may not be individuals who are involved in the kernel development
> scene to any great extent, but I am sure that remap_file_pages() being
> deprecated would seriously piss off a lot of individuals. The pattern
> has even had a section in the Wikipedia article for quite some time:
> http://en.wikipedia.org/wiki/Circular_buffer#Mirroring

I believe remap_file_pages() is abused here. But it seems we will have to
keep emulation in place for a long time.

> 
> It would be most preferable from a user standpoint to keep the
> existing system intact, 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.

-- 
 Kirill A. Shutemov