2.6.20 kernel hang with USB drive and vfat doing ftruncate

[Kumar Gala <galak@kernel.crashing.org>]

I'm seeing an issue with a stock 2.6.20 kernel running on an embedded  
PPC.  I've got a usb flash drive plugged in and the filesystem on the  
drive is vfat.  Running with 64M and no swap.

If I execute a series of large (100M+) ftruncate() on the disk the  
kernel will hang and never return.  It seems to be stuck in the idle  
loop().

The following is the test program I'm running:

#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>

void usage (void)
{
         printf ("truncate_test <filename> <size>\n\n");
}

int main(int argc, char *argv[])
{
         int fd, i;
         int ret = 0;
         unsigned int len;

         if (argc != 3) {
                 printf("Invalid number of arguments\n\n");
                 usage();
                 exit(1);
         }

         fd = open(argv[1], O_CREAT|O_RDWR|O_TRUNC, S_IRWXU);
         len = strtoul(argv[2], NULL, 0);

         ret = ftruncate(fd, len);

         if (ret)
                 printf ("ftruncate ret = %d %d\n", ret, errno);

         close(fd);

         return ret;
}

I usually run the following twice to get the hang state:

time ./trunc_test bar 100000000 &
time ./trunc_test baz 100000000 &

I was wondering if anyone had any suggestions on what to poke at next  
to try and figure out what is going on.

- k