On Mon, May 15, 2006 at 10:06:29PM +0200, Michael Kerrisk wrote:
> > > > changes:
> > > > make mmap.2 reference mincore
> > >
> > > Done, for 2.33.
> > >
> > > > explicitly state that a read-only MAP_PRIVATE is equivalent to
> > > > MAP_SHARED
> > >
> > > *why?* You do not explain what benefit this serves.
> > Because, upon reflection, it isn't immediately clear from the wording
> > of the page as it exists now. In particular:
> >
> > Create a private copy-on-write mapping
>
> Umm -- we were talking about read-only mappings?
Yes; I wish to make it clear that, for a read-only map, the behaviour
is the same.
> > This could be interpretted as meaning "create a new mapping, even if
> > the file is already mapped", which isn't what people usually want, and
> > isn't what it does anyway..
> >
> > > > An alternative would be to take the commented-out Linus quote
> > > > embedded
> > > > in mmap.2 which says precisely what I want and create a NOTES section
> > > > out of it.
>
> Let me put things another way: why should a programmer care
> about whether read-only mappings are done the same way
> internally for both MAP_SHARED and MAP_PRIVATE?
A programmer should care whether using MAP_PRIVATE uses extra
resources than a MAP_SHARED (both maps presumably have some
overhead, for the initial mapping and for each additional mapping). I
question whether this is presently as clear as it should be.
I think it is typical to need to have *some* understanding of a lower
layer interface than one uses directly, even if the full
implementation details are deliberately opaque.
Would it be better to change the descriptions of MAP_{PRIVATE,SHARED}
themselves? If you think so, I can suggest something.
Justin
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