Re: [PATCH] Fix random fast-import errors when compiled with NO_MMAP

On Thu, 17 Jan 2008, Shawn O. Pearce wrote:

fast-import was relying on the fact that on most systems mmap() and
write() are synchronized by the filesystem's buffer cache.  We were
relying on the ability to mmap() 20 bytes beyond the current end
of the file, then later fill in those bytes with a future write()
call, then read them through the previously obtained mmap() address.

This isn't always true with some implementations of NFS, but it is
especially not true with our NO_MMAP=YesPlease build time option used
on some platforms.

In fact, even with mmap(), it's not guaranteed. There are really crappy 
mmap implementations out there, partly due to bad CPU design (virtual CPU 
caches without coherency), but more often due to total crap OS.

(Yeah, Linux did count in that area at some point. Long ago. Early 90's. 
Maybe)

I think HP-UX used to have non-coherent mmap for the longest time, due to 
carrying around some totally crap memory management based on some ancient 
BSD version that everybody else (including the BSD's) had long since 
jettisoned.

That said, I suspect any unix you can run today (without calling it a 
retro setup) probably has coherent-enough mmap. The possible virtual cache 
coherency issue is unlikely to be able to trigger this (and not relevant 
on any sane hardware anyway).

				Linus