Re: [PATCH v2 5/5] dax: handle media errors in dax_do_io

[Jeff Moyer]

"Verma, Vishal L" <vishal.l.verma@xxxxxxxxx> writes:

> On Fri, 2016-04-15 at 12:11 -0400, Jeff Moyer wrote:
>> Vishal Verma <vishal.l.verma@xxxxxxxxx> writes:
>> > +	if (IS_DAX(inode)) {
>> > +		ret = dax_do_io(iocb, inode, iter, offset,
>> > blkdev_get_block,
>> > Â				NULL, DIO_SKIP_DIO_COUNT);
>> > -	return __blockdev_direct_IO(iocb, inode, I_BDEV(inode),
>> > iter, offset,
>> > +		if (ret == -EIO && (iov_iter_rw(iter) == WRITE))
>> > +			ret_saved = ret;
>> > +		else
>> > +			return ret;
>> > +	}
>> > +
>> > +	ret = __blockdev_direct_IO(iocb, inode, I_BDEV(inode),
>> > iter, offset,
>> > Â				ÂÂÂÂblkdev_get_block, NULL, NULL,
>> > Â				ÂÂÂÂDIO_SKIP_DIO_COUNT);
>> > +	if (ret < 0 && ret_saved)
>> > +		return ret_saved;
>> > +
>> Hmm, did you just break async DIO?ÂÂI think you did!ÂÂ:)
>> __blockdev_direct_IO can return -EIOCBQUEUED, and you've now turned
>> that
>> into -EIO.ÂÂReally, I don't see a reason to save that first
>> -EIO.ÂÂThe
>> same applies to all instances in this patch.
>
> The reason I saved it was if __blockdev_direct_IO fails for some
> reason, we should return the original cause o the error, which was an
> EIO.. i.e. we shouldn't be hiding the EIO if the direct_IO fails with
> something else..

OK.

> But, how does _EIOCBQUEUED work? Maybe we need an exception for it?

For async direct I/O, only the setup phase of the I/O is performed and
then we return to the caller.  -EIOCBQUEUED signifies this.

You're heading towards code that looks like this:

        if (IS_DAX(inode)) {
                ret = dax_do_io(iocb, inode, iter, offset, blkdev_get_block,
                                NULL, DIO_SKIP_DIO_COUNT);
                if (ret == -EIO && (iov_iter_rw(iter) == WRITE))
                        ret_saved = ret;
                else
                        return ret;
        }

        ret = __blockdev_direct_IO(iocb, inode, I_BDEV(inode), iter, offset,
                                    blkdev_get_block, NULL, NULL,
                                    DIO_SKIP_DIO_COUNT);
        if (ret < 0 && ret != -EIOCBQUEUED && ret_saved)
                return ret_saved;

There's a lot of special casing here, so you might consider adding
comments.

Cheers,
Jeff