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

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Date:	Fri, 15 Apr 2016 17:37:02 +0000
From:	"Verma, Vishal L" <vishal.l.verma@...el.com>
To:	"jmoyer@...hat.com" <jmoyer@...hat.com>
CC:	"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>,
	"xfs@....sgi.com" <xfs@....sgi.com>,
	"linux-nvdimm@...1.01.org" <linux-nvdimm@...1.01.org>,
	"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>,
	"Wilcox, Matthew R" <matthew.r.wilcox@...el.com>,
	"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 13:11 -0400, Jeff Moyer wrote:
> "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.

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