On Mon, Aug 25, 2025 at 02:07:15PM +0200, Jan Kara wrote:
> On Fri 22-08-25 18:57:08, Ritesh Harjani wrote:
> > Keith Busch <kbusch@xxxxxxxx> writes:
> > >
> > > - EXT4 falls back to buffered io for writes but not for reads.
> >
> > ++linux-ext4 to get any historical context behind why the difference of
> > behaviour in reads v/s writes for EXT4 DIO.
>
> Hum, how did you test? Because in the basic testing I did (with vanilla
> kernel) I get EINVAL when doing unaligned DIO write in ext4... We should be
> falling back to buffered IO only if the underlying file itself does not
> support any kind of direct IO.
Simple test case (dio-offset-test.c) below.
I also ran this on vanilla kernel and got these results:
# mkfs.ext4 /dev/vda
# mount /dev/vda /mnt/ext4/
# make dio-offset-test
# ./dio-offset-test /mnt/ext4/foobar
write: Success
read: Invalid argument
I tracked the "write: Success" down to ext4's handling for the "special"
-ENOTBLK error after ext4_want_directio_fallback() returns "true".
dio-offset-test.c:
---
#ifndef _GNU_SOURCE
#define _GNU_SOURCE
#endif
#include <sys/uio.h>
#include <err.h>
#include <errno.h>
#include <fcntl.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
int main(int argc, char **argv)
{
unsigned int pagesize;
struct iovec iov[2];
int ret, fd;
void *buf;
if (argc < 2)
err(EINVAL, "usage: %s <file>", argv[0]);
pagesize = sysconf(_SC_PAGE_SIZE);
ret = posix_memalign((void **)&buf, pagesize, 2 * pagesize);
if (ret)
err(errno, "%s: failed to allocate buf", __func__);
fd = open(argv[1], O_RDWR | O_CREAT | O_TRUNC | O_DIRECT);
if (fd < 0)
err(errno, "%s: failed to open %s", __func__, argv[1]);
iov[0].iov_base = buf;
iov[0].iov_len = 256;
iov[1].iov_base = buf + pagesize;
iov[1].iov_len = 256;
ret = pwritev(fd, iov, 2, 0);
perror("write");
ret = preadv(fd, iov, 2, 0);
perror("read");
return 0;
}
--
