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


On Fri, 19 Nov 2004, David Howells wrote:
>
> > > 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.
>
> I think it is a requirement. With normal Linux, if you do a write that crosses
> a shared mmap it will change the mmapped data.

Indeed.

Some other UNIXes are broken in this regard, and there are cache coherency
issues on some platforms that people who depend on this behaviour may need
to be aware of, but in general Linux has always gone for coherenct mmap's.

POSIX doesn't require it, but quite frankly, non-coherent shared mmap just
isn't worth it. You might as well not support it at all at that point.

[ I think at least HP-UX used to be broken in this regard, but HP-UX had
such atrocious mmap behaviour _anyway_ that I don't understand how
anybody could use it (you couldn't munmap partial mappings etc). I think
they finally fixed it a few years ago and updated their source base from
some totally ancient BSD to something a bit more modern. ]

Linus
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