Re: [PATCH] ext4: Properly sync file size update after O_SYNC direct IO

[Jan Kara <jack@suse.cz>]

On Thu 12-10-23 11:26:15, Dave Chinner wrote:
> On Wed, Oct 11, 2023 at 04:21:55PM +0200, Jan Kara wrote:
> > Gao Xiang has reported that on ext4 O_SYNC direct IO does not properly
> > sync file size update and thus if we crash at unfortunate moment, the
> > file can have smaller size although O_SYNC IO has reported successful
> > completion. The problem happens because update of on-disk inode size is
> > handled in ext4_dio_write_iter() *after* iomap_dio_rw() (and thus
> > dio_complete() in particular) has returned and generic_file_sync() gets
> > called by dio_complete(). Fix the problem by handling on-disk inode size
> > update directly in our ->end_io completion handler.
> > 
> > References: https://lore.kernel.org/all/02d18236-26ef-09b0-90ad-030c4fe3ee20@linux.alibaba.com
> > Reported-by: Gao Xiang <hsiangkao@linux.alibaba.com>
> > Signed-off-by: Jan Kara <jack@suse.cz>
> > ---
> >  fs/ext4/file.c | 139 ++++++++++++++++++-------------------------------
> >  1 file changed, 52 insertions(+), 87 deletions(-)
> .....
> > @@ -388,9 +342,28 @@ static ssize_t ext4_handle_inode_extension(struct inode *inode, loff_t offset,
> >  		 */
> >  		if (inode->i_nlink)
> >  			ext4_orphan_del(NULL, inode);
> > +		return;
> >  	}
> > +	/*
> > +	 * If i_disksize got extended due to writeback of delalloc blocks while
> > +	 * the DIO was running we could fail to cleanup the orphan list in
> > +	 * ext4_handle_inode_extension(). Do it now.
> > +	 */
> > +	if (!list_empty(&EXT4_I(inode)->i_orphan) && inode->i_nlink) {
> > +		handle_t *handle = ext4_journal_start(inode, EXT4_HT_INODE, 2);
> 
> So this has to be called after the DIO write completes and calls
> ext4_handle_inode_extension()?

Yes, if the write was setup as extending one ('extend' is set to true in
ext4_dio_write_iter()).

> > @@ -606,9 +570,8 @@ static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
> >  			   dio_flags, NULL, 0);
> >  	if (ret == -ENOTBLK)
> >  		ret = 0;
> > -
> >  	if (extend)
> > -		ret = ext4_handle_inode_extension(inode, offset, ret, count);
> > +		ext4_inode_extension_cleanup(inode, ret);
> 
> Because this doesn't wait for AIO DIO to complete and actually
> extend the file before running the cleanup code...

As Gao wrote, ext4 sets IOMAP_DIO_FORCE_WAIT if 'extend' is set (see
ext4_dio_write_checks()) so if we get to calling
ext4_inode_extension_cleanup() we are guaranteed the IO has already
completed.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR