On Tue 26-08-25 10:29:58, Ritesh Harjani wrote:

Keith Busch [off-list ref] writes:

quoted

On Mon, Aug 25, 2025 at 02:07:15PM +0200, Jan Kara wrote:

quoted

On Fri 22-08-25 18:57:08, Ritesh Harjani wrote:

quoted

Keith Busch [off-list ref] writes:

quoted

  - 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. 

Right. And the fallback for writes was actually inadvertedly "added" by
commit bc264fea0f6f "iomap: support incremental iomap_iter advances". That
changed the error handling logic. Previously if iomap_dio_bio_iter()
returned EINVAL, it got propagated to userspace regardless of what
->iomap_end() returned. After this commit if ->iomap_end() returns error
(which is ENOTBLK in ext4 case), it gets propagated to userspace instead of
the error returned by iomap_dio_bio_iter().

Now both the old and new behavior make some sense so I won't argue that the
new iomap_iter() behavior is wrong. But I think we should change ext4 back
to the old behavior of failing unaligned dio writes instead of them falling
back to buffered IO. I think something like the attached patch should do
the trick - it makes unaligned dio writes fail again while writes to holes
of indirect-block mapped files still correctly fall back to buffered IO.
Once fstests run completes, I'll do a proper submission...


								Honza
-- 
Jan Kara [off-list ref]
SUSE Labs, CR