Re: First cut at git port to Cygwin

From: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Date: 2005-10-11 05:42:49
Hi,

On Mon, 10 Oct 2005, H. Peter Anvin wrote:

> Johannes Schindelin wrote:
> > 
> > > Junio C Hamano wrote:
> > > 
> > > > PROT_WRITE is true, but we do MAP_PRIVATE, and if I recall
> > > > correctly we do not write file via mmap -- at least we do not
> > > > intend to.
> > > > 
> > > 
> > > Then PROT_READ probably makes more sense?
> > 
> > Not necessarily. Sometimes you need to annotate the data from the index, and
> > this does not need to be written back to the index file.
> > 
> 
> In the above sentence, emphasis on "at least we do not intend to."  If writes
> are done legitimately then that's fine, but we shouldn't have "accidental
> writes" -- those would be program bugs!

Yes, those would be bugs. However, if I understood the man page for mmap() 
correctly, then PROT_WRITE && MAP_PRIVATE makes the data copy-on-write, 
which means that those bugs would have been found (because the changes 
would no longer be present when git was called the next time). And I 
checked: all mmap() calls in git are MAP_PRIVATE.

> > > > Yes.  It might have been overkill that you supported writing
> > > > changes back, though.
> > > 
> > > Not just overkill; if we do MAP_PRIVATE it's actively WRONG.
> > 
> > See above.
> > 
> 
> Eh?  If we MAP_PRIVATE, *and* we (intentionally) write to it, we *BETTER* not
> write anything back.

Yes. That was *my* mistake.

Ciao,
Dscho
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Received on Tue Oct 11 05:43:18 2005

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