linux-kernel - Re: [PATCHv3 0/8] direct-io: even more flexible io vectors

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Message-ID: <aKx485EMthHfBWef@kbusch-mbp>
Date: Mon, 25 Aug 2025 08:53:39 -0600
From: Keith Busch <kbusch@...nel.org>
To: Jan Kara <jack@...e.cz>
Cc: Ritesh Harjani <ritesh.list@...il.com>, Keith Busch <kbusch@...a.com>,
	linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org,
	snitzer@...nel.org, axboe@...nel.dk, dw@...idwei.uk,
	brauner@...nel.org, hch@....de, martin.petersen@...cle.com,
	djwong@...nel.org, linux-xfs@...r.kernel.org,
	viro@...iv.linux.org.uk, Jan Kara <jack@...e.com>
Subject: Re: [PATCHv3 0/8] direct-io: even more flexible io vectors

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@...a.com> 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;
}
--