Re: [PATCH 17/20] FRV: Better mmap support in uClinux

[Matt Mackall]

On Fri, Nov 19, 2004 at 04:26:43PM +0000, David Howells wrote:
> 
> > >  (3) Files (and blockdevs) cannot be mapped shared since it is not
> > >  really possible to honour this by writing any changes back to the
> > >  backing device.
> > 
> > [way behind on email]
> > 
> > I think this could be done at msync, munmap and exit time? You end up
> > flushing the entire mapping, but it's still correct (and POSIX
> > compliant).
> 
> Don't forget write() too. If someone does a write, that would have to be
> written over the mapping too. Obviously this is not impossible.

I don't see such a requirement, but it'd be nice, yes.
 
> > And, if you wanted to be really clever, you could store a hash of each
> > page to detect changes and avoid the extra I/O.
> 
> It'd probably have to be something like an md5 sum.

Arguably, it needn't be cryptographically strong. But that's another
discussion.

> Okay, technically, we could probably emulate it, but is it worth it? I think
> it's something to bear in mind for another time.

Well I wasn't volunteering, just pointing out it's not as hard as
claimed. Thankfully all the boxes I currently have to care about are
not so special.

-- 
Mathematics is the supreme nostalgia of our time.
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