Keith Busch <kbusch@xxxxxxxxxx> writes:
> 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".
>
Right. Ext4 has fallback only for dio writes but not for DIO reads...
buffered
static inline bool ext4_want_directio_fallback(unsigned flags, ssize_t written)
{
/* must be a directio to fall back to buffered */
if ((flags & (IOMAP_WRITE | IOMAP_DIRECT)) !=
(IOMAP_WRITE | IOMAP_DIRECT))
return false;
...
}
So basically the path is ext4_file_[read|write]_iter() -> iomap_dio_rw
-> iomap_dio_bio_iter() -> return -EINVAL. i.e. from...
if ((pos | length) & (bdev_logical_block_size(iomap->bdev) - 1) ||
!bdev_iter_is_aligned(iomap->bdev, dio->submit.iter))
return -EINVAL;
EXT4 then fallsback to buffered-io only for writes, but not for reads.
-ritesh
> 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;
> }
> --
