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

[Verma, Vishal L]

On Fri, 2016-04-15 at 13:11 -0400, Jeff Moyer wrote:
> "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.ÂÂ-EIOCBQUEUED signifies this.
> 
> You're heading towards code that looks like this:
> 
> ÂÂÂÂÂÂÂÂif (IS_DAX(inode)) {
> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂret = dax_do_io(iocb, inode, iter, offset,
> blkdev_get_block,
> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂNULL, DIO_SKIP_DIO_COUNT);
> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂif (ret == -EIO && (iov_iter_rw(iter) == WRITE))
> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂret_saved = ret;
> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂelse
> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂreturn ret;
> ÂÂÂÂÂÂÂÂ}
> 
> ÂÂÂÂÂÂÂÂret = __blockdev_direct_IO(iocb, inode, I_BDEV(inode), iter,
> offset,
> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂblkdev_get_block, NULL, NULL,
> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂDIO_SKIP_DIO_COUNT);
> ÂÂÂÂÂÂÂÂif (ret < 0 && ret != -EIOCBQUEUED && ret_saved)
> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂreturn ret_saved;
> 
> There's a lot of special casing here, so you might consider adding
> comments.

Correct - maybe we should reconsider wrapper-izing this? :)

Thanks for the explanation and for catching this. I'll fix it for the
next revision.

> 
> Cheers,
> Jeff