linux-ext4 - Re: [PATCH v2 5/5] dax: handle media errors in dax_do

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Date:	Fri, 15 Apr 2016 16:54:48 +0000
From:	"Verma, Vishal L" <vishal.l.verma@...el.com>
To:	"jmoyer@...hat.com" <jmoyer@...hat.com>
CC:	"Wilcox, Matthew R" <matthew.r.wilcox@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
	"hch@...radead.org" <hch@...radead.org>,
	"linux-nvdimm@...1.01.org" <linux-nvdimm@...1.01.org>,
	"xfs@....sgi.com" <xfs@....sgi.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"axboe@...com" <axboe@...com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"david@...morbit.com" <david@...morbit.com>,
	"jack@...e.cz" <jack@...e.cz>
Subject: Re: [PATCH v2 5/5] dax: handle media errors in dax_do_io

On Fri, 2016-04-15 at 12:11 -0400, Jeff Moyer wrote:
> Vishal Verma <vishal.l.verma@...el.com> writes:
> 
> > 
> > dax_do_io (called for read() or write() for a dax file system) may
> > fail
> > in the presence of bad blocks or media errors. Since we expect that
> > a
> > write should clear media errors on nvdimms, make dax_do_io fall
> > back to
> > the direct_IO path, which will send down a bio to the driver, which
> > can
> > then attempt to clear the error.
> [snip]
> 
> > 
> > +	if (IS_DAX(inode)) {
> > +		ret = dax_do_io(iocb, inode, iter, offset,
> > blkdev_get_block,
> >  				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,
> >  				    blkdev_get_block, NULL, NULL,
> >  				    DIO_SKIP_DIO_COUNT);
> > +	if (ret < 0 && ret_saved)
> > +		return ret_saved;
> > +
> Hmm, did you just break async DIO?  I think you did!  :)
> __blockdev_direct_IO can return -EIOCBQUEUED, and you've now turned
> that
> into -EIO.  Really, I don't see a reason to save that first
> -EIO.  The
> 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..
But, how does _EIOCBQUEUED work? Maybe we need an exception for it? 

Thanks,
	-Vishal