read() MAX_IO_SIZE bytes, more than SSIZE_MAX?

Hi there

While investigating the problem with hung git-upload-pack we think to have 
found a bug in wrapper.c:

#define MAX_IO_SIZE (8*1024*1024)

This is then used in xread() to split read()s into suitable chunks.
So far so good, but read() is only guaranteed to read as much as SSIZE_MAX 
bytes at a time. And on our platform that is way lower than those 8MB (only 
52kB, POSIX allows it to be as small as 32k), and as a (rather strange) 
consequence mmap() (from compat/mmap.c) fails with EACCESS (why EACCESS?), 
because xpread() returns something > 0.

How large is SSIZE_MAX on other platforms? What happens there if you try to 
read() more? Should't we rather use SSIZE_MAX on all platforms? If I'm 
reading the header files right, on Linux it is LONG_MAX (2TB?), so I guess 
we should really go for MIN(8*1024*1024,SSIZE_MAX)?


bye, Jojo