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

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Date:	Fri, 25 Mar 2016 14:42:37 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	"Verma, Vishal L" <vishal.l.verma@...el.com>
Cc:	"hch@...radead.org" <hch@...radead.org>,
	"linux-block@...r.kernel.org" <linux-block@...r.kernel.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>,
	"axboe@...com" <axboe@...com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"ross.zwisler@...ux.intel.com" <ross.zwisler@...ux.intel.com>,
	"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 5/5] dax: handle media errors in dax_do_io

On Fri, Mar 25, 2016 at 1:59 PM, Verma, Vishal L
<vishal.l.verma@...el.com> wrote:
> On Fri, 2016-03-25 at 03:45 -0700, Christoph Hellwig wrote:
>> On Thu, Mar 24, 2016 at 05:17:30PM -0600, Vishal Verma wrote:
>> >
>> > 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.
>> Leave the fallback on -EIO to the callers please.  They generally
>> call
>> __blockdev_direct_IO anyway, so it should actually become simpler
>> that
>> way.
>
> I thought of this, but made the retrying happen in the wrapper so that
> it can be centralized. If the callers were to become responsible for
> the retry, then any new callers of dax_do_io might not realize they are
> responsible for retrying, and hit problems.

That's their prerogative otherwise you are precluding an alternate
handling of a dax_do_io() failure.  Maybe a fs or upper layer can
recover in a different manner than re-submit the I/O to the
__blockdev_direct_IO path.
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