Re: [PATCH] Alternate futex non-page-pinning and COW fix

[Linus Torvalds]

On Wed, 3 Sep 2003, Hugh Dickins wrote:
> 
> Of course (not).  That's the point, they do work on private mappings, but
> the semantics are different on private mappings from on shared mappings:
> on private mappings they're private to the mm, on shared mappings they're
> shared with other mms (via the shared file).

That's not true. It never has been true in Linux.

Private mappings that haven't been broken by COW (and a read-only mapping
never will be) will see updates as they happen on the file that backs it.
That's the fundamental difference between "mmap(MAP_PRIVATE)" and
"read()".

You may not like it, and others too have not liked it (Hurd and Mach do 
this big dance about MAP_COPY that really creates a static _copy_ of the 
state at the time of the mmap), but it's just a fact.

Repeat after me: private read-only mappings are 100% equivalent to shared
read-only mappings. No ifs, buts, or maybes. This is a FACT. It's a fact 
codified in many years of Linux implementation, but it's a fact outside of 
that too.

(Yeah, yeah, I know some broken old Unixes do not offer mmap consistency
guarantees, and nntpd is unhappy. But Linux isn't broken.)

			Linus

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