Re: [PATCH 4/8] btrfs: Switch to iomap_dio_rw() for dio

[Johannes Thumshirn <jthumshirn@suse.de>]

On Thu, Dec 05, 2019 at 09:19:59AM -0800, Christoph Hellwig wrote:
> I actually much prefer exporting generic_file_buffered_read and will
> gladly switch other callers not needing the messy direct I/O handling
> in generic_file_read_iter over to generic_file_buffered_read once this
> series is merged.

I think you misunderstood me here, I meant the code to be:

static ssize_t btrfs_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
{
	ssize_t ret = 0;

	if (iocb->ki_flags & IOCB_DIRECT) {
		struct inode *inode = file_inode(iocb->ki_filp);

		inode_lock_shared(inode);
		ret = btrfs_direct_IO(iocb, to);
		inode_unlock_shared(inode);
		if (ret < 0)
			return ret;
		}
	}

	return generic_file_read_iter(icob, to);
}

This way an iocb that is no dio will end in generic_file_read_iter():

generic_file_read_iter(iocb, to)
{
	size_t count = iov_iter_count(iter);
        ssize_t retval = 0;

        if (!count)
                goto out; /* skip atime */

	if (iocb->ki_flags & IOCB_DIRECT) {
		skipped as flag is not set
	}

	retval = generic_file_buffered_read(iocb, iter, retval);
out:
	return retval;
}

Meaning we do not need to export generic_file_buffered_read() and still can
skip the generic DIO madness.

Makes sense?

	Johannes