Red Hat Bugzilla – Bug 72242
O_DIRECT flag in open() syscall seems to be not working
Last modified: 2007-04-18 12:45:48 EDT
From Bugzilla Helper: User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0) Description of problem: I'm trying to use Direct I/O in my program by I'm not able to use O_DIRECT flag in open() syscall. Version-Release number of selected component (if applicable): How reproducible: Always Steps to Reproduce: #include <sys/types.h> #include <sys/stat.h> #include <errno.h> #include <fcntl.h> #include <stdio.h> #include <stdlib.h> #include <string.h> #include <unistd.h> #define O_DIRECT 040000 #define BUFSIZE 8192 int main (int argc, char **argv) { int fd, err; char *buf; if ( argc < 2 ) { printf ("Usage: readttst <filename>\n"); exit (1); } if ( (buf=valloc(BUFSIZE)) == NULL ) { printf ("Cannot allocate enough memory\n"); exit (4); } fd = open (argv[1], O_RDONLY | O_DIRECT); if ( fd == -1 ) { printf ("Cannot open file: %s. Error: %s\n", argv[1], strerror(errno)); exit (2); } if ( (err = read(fd, buf, BUFSIZE)) != 0 ) { if ( err == -1 ) { printf ("Error reading file: %s\n", strerror(errno)); exit (3); } printf ("OK"); } close (fd); return 0; } Actual Results: Program writes: Error reading file: Invalid argument Expected Results: I should get many "OK" messages. Additional info:
what filesystem is this on?
Filesystems are ext2 and ext3. Program succeeds on /proc though.
I just tried this on a 7.3 system, and it works fine for ext2 and fails on ext3, as expected --- ext3 does not yet support O_DIRECT.
# mount /dev/cciss/c0d0p5 on / type ext3 (rw) none on /proc type proc (rw) usbdevfs on /proc/bus/usb type usbdevfs (rw) none on /dev/pts type devpts (rw,gid=5,mode=620) none on /dev/shm type tmpfs (rw) /dev/cciss/c0d0p6 on /var type ext3 (rw) /dev/cciss/c0d0p1 on /boot type ext2 (rw) # ./readtst /boot/vmlinuz-2.4.18-5smp Error reading file: Invalid argument
Which kernel (uname -a), and what does "cat /proc/mounts" show?
# uname -a Linux antares 2.4.18-5smp #1 SMP Mon Jun 10 15:19:40 EDT 2002 i686 unknown # cat /proc/mounts rootfs / rootfs rw 0 0 /dev/root / ext3 rw 0 0 /proc /proc proc rw 0 0 usbdevfs /proc/bus/usb usbdevfs rw 0 0 none /dev/pts devpts rw 0 0 none /dev/shm tmpfs rw 0 0 /dev/cciss/c0d0p6 /var ext3 rw 0 0 /dev/cciss/c0d0p1 /boot ext2 rw 0 0
My mistake. O_DIRECT produces errors only when trying to read not full block files' endings. And where to read more about programming with O_DIRECT? Which filesystems supports it, what are the possible block sizes and so on. [root@antares root]# mke2fs /dev/cciss/c0d0p2 mke2fs 1.27 (8-Mar-2002) Filesystem label= OS type: Linux Block size=4096 (log=2) Fragment size=4096 (log=2) 256000 inodes, 511020 blocks 25551 blocks (5.00%) reserved for the super user First data block=0 16 block groups 32768 blocks per group, 32768 fragments per group 16000 inodes per group Superblock backups stored on blocks: 32768, 98304, 163840, 229376, 294912 Writing inode tables: done Writing superblocks and filesystem accounting information: done This filesystem will be automatically checked every 32 mounts or 180 days, whichever comes first. Use tune2fs -c or -i to override. [root@antares root]# mount -t ext2 /dev/cciss/c0d0p2 /mnt/cdrom/ [root@antares root]# cd /mnt/cdrom/ [root@antares cdrom]# dd if=/dev/zero of=testing bs=8192 count=1024 1024+0 records in 1024+0 records out [root@antares cdrom]# ~/readtst testing OKOKOKOKOKOKOKOKOKOKOKOKOK<and so on> [root@antares cdrom]# cp /boot/vmlinux-2.4.18-5smp . [root@antares cdrom]# ~/readtst vmlinux-2.4.18-5smp OKOKOKOKOKOKOKOKOK<.....>OKOKOKOKOKOKOKOKOKError reading file: Invalid argument
O_DIRECT _requires_ page size sized and aligned IO. Anything else is invalid by definition.