Subject: Re: read() MAX_IO_SIZE bytes, more than SSIZE_MAX?

From: Torsten Bögershausen <[email-masked]>
Date: Sat, 07 Feb 2015 18:19:53 +0100

On 2015-02-07 17.45, Joachim Schmitz wrote:
> 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)?

How about changing wrapper.c like this:

#ifndef MAX_IO_SIZE
 #define MAX_IO_SIZE (8*1024*1024)
#endif
---------------------
and to change config.mak.uname like this:

ifeq ($(uname_S),NONSTOP_KERNEL)

	BASIC_CFLAGS += -DMAX_IO_SIZE=(32*1024)
Does this work for you ?