Messages in this thread | | | Date | Wed, 3 Sep 2003 12:05:36 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: [PATCH] Alternate futex non-page-pinning and COW fix |
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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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