Re: First cut at git port to Cygwin

From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Oct 10, 2005, 19:42 UTC
Message-ID: <Pine.LNX.4.63.0510102139540.7861@wbgn013.biozentrum.uni-wuerzburg.de>
In-Reply-To: <434AC058.60803@zytor.com>

Hi,

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

Show 18 quoted lines

> 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.

Show 10 quoted lines

> > > > 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