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

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Date:	Fri, 15 Apr 2016 13:11:17 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	"Verma\, Vishal L" <vishal.l.verma@...el.com>
Cc:	"Wilcox\, Matthew R" <matthew.r.wilcox@...el.com>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-block\@vger.kernel.org" <linux-block@...r.kernel.org>,
	"hch\@infradead.org" <hch@...radead.org>,
	"linux-nvdimm\@ml01.01.org" <linux-nvdimm@...1.01.org>,
	"xfs\@oss.sgi.com" <xfs@....sgi.com>,
	"linux-mm\@kvack.org" <linux-mm@...ck.org>,
	"viro\@zeniv.linux.org.uk" <viro@...iv.linux.org.uk>,
	"akpm\@linux-foundation.org" <akpm@...ux-foundation.org>,
	"axboe\@fb.com" <axboe@...com>,
	"linux-fsdevel\@vger.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-ext4\@vger.kernel.org" <linux-ext4@...r.kernel.org>,
	"david\@fromorbit.com" <david@...morbit.com>,
	"jack\@suse.cz" <jack@...e.cz>
Subject: Re: [PATCH v2 5/5] dax: handle media errors in dax_do_io

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

> On Fri, 2016-04-15 at 12:11 -0400, Jeff Moyer wrote:
>> Vishal Verma <vishal.l.verma@...el.com> writes:
>> > +	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..

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
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