Description of problem:

Hi,

I am using O_DIRECT to read regular files under ext3, with no special
mount options.  It works fine except for the final read when the
remaining file size is less than pagesize.  In this case 0 byte is
returned instead of the actual number of bytes.

The program I used is diotest.c:

#include <stdio.h>
#include <fcntl.h>
#include <errno.h>

main()
{
    char *message; 
    int fd = open("/work/xiaojun/oxx", O_RDONLY|O_DIRECT); 
    int len, pagesize = getpagesize(); 

    printf("pagesize = %d.\n", pagesize);

    posix_memalign((void **)&message, pagesize, pagesize); 

    if(fd < 0)
    { 
        printf("Unable to open file, errno is %d.\n", errno); 
    }
    else
    { 

        while((len = read(fd, message, pagesize)) > 0)
        {
            printf("%d bytes read from file.\n", len); 
        }

        if(len < 0)
        { 
            perror("read"); 
        }
        else
        { 
            printf("%d bytes read from file.\n", len); 
        } 
    } 
    close(fd); 
}

The file /work/xiaojun/oxx is 5587 bytes. 


Version-Release number of selected component (if applicable):
kernel-2.4.21-20.0.1Elsmp


How reproducible:
Every time


Steps to Reproduce:
1. Modify diotest.c to change file name to an existing file with size
bigger than 4K but less than 8K.
2. gcc -D_GNU_SOURCE diotest.c
3. ./a.out
4. For comparison, remove the O_DIRECT flag from open() and repeat 
steps 2 and 3 above.

  
Actual results:
pagesize = 4096.
4096 bytes read from file.
0 bytes read from file.


Expected results:
pagesize = 4096.
4096 bytes read from file.
1491 bytes read from file.
0 bytes read from file.


Additional info: