I tested this with a number of kernel versions from 2.6.27 to 2.6.31.
All of them reproduced this problem. I checked this because strace
shows 'git add' doing a mmap(..., MAP_PRIVATE, ...) of its input file,
so I was wondering if there might have been a recent change in mmap()
behavior in either git or the kernel.
From mmap(2): "it is unspecified whether changes made to the file after
the mmap() call are visible in the mapped region".
You may think that doing a dummy "*p = *p" every 4096 bytes `fixes' it
(because it causes copy-on-write for every page) but even that does not
work because you can get a SIGBUS if the file is truncated while you
have it mapped privately (e.g. by fopen ("file", "w") or open with O_TRUNC).
Testcase:
#include <sys/mman.h>
#include <fcntl.h>
#include <stdio.h>
int
main ()
{
system ("echo foo > file.test");
int f = open ("file.test", O_RDONLY);
char *p = mmap (NULL, 4096, PROT_READ, MAP_PRIVATE, f, 0);
close (f);
printf ("%s", p); // prints "foo\n"
f = open ("file.test", O_RDWR | O_CREAT | O_TRUNC, 0666);
write (f, "bar\n", 4); // comment out and next printf SIGSEGVs
printf ("%s", p); // prints "bar\n"
}
This means that MAP_PRIVATE is utterly useless.
Paolo
--
To unsubscribe from this list: send the line "unsubscribe git" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html