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

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Date:	Fri, 15 Apr 2016 14:24:10 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	"Verma\, Vishal L" <vishal.l.verma@...el.com>,
	"hch\@infradead.org" <hch@...radead.org>,
	"jack\@suse.cz" <jack@...e.cz>, "axboe\@fb.com" <axboe@...com>,
	"linux-nvdimm\@ml01.01.org" <linux-nvdimm@...1.01.org>,
	"david\@fromorbit.com" <david@...morbit.com>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"xfs\@oss.sgi.com" <xfs@....sgi.com>,
	"linux-block\@vger.kernel.org" <linux-block@...r.kernel.org>,
	"linux-mm\@kvack.org" <linux-mm@...ck.org>,
	"viro\@zeniv.linux.org.uk" <viro@...iv.linux.org.uk>,
	"linux-fsdevel\@vger.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"akpm\@linux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-ext4\@vger.kernel.org" <linux-ext4@...r.kernel.org>,
	"Wilcox\, Matthew R" <matthew.r.wilcox@...el.com>
Subject: Re: [PATCH v2 5/5] dax: handle media errors in dax_do_io

Dan Williams <dan.j.williams@...el.com> writes:

> On Fri, Apr 15, 2016 at 11:06 AM, Jeff Moyer <jmoyer@...hat.com> wrote:
>> Dan Williams <dan.j.williams@...el.com> writes:
>>
>>>>> There's a lot of special casing here, so you might consider adding
>>>>> comments.
>>>>
>>>> Correct - maybe we should reconsider wrapper-izing this? :)
>>>
>>> Another option is just to skip dax_do_io() and this special casing
>>> fallback entirely if errors are present.  I.e. only attempt dax_do_io
>>> when: IS_DAX() && gendisk->bb && bb->count == 0.
>>
>> So, if there's an error anywhere on the device, penalize all I/O (not
>> just writes, and not just on sectors that are bad)?  I'm not sure that's
>> a great plan, either.
>>
>
> If errors are rare how much are we actually losing in practice?

How long is a piece of string?

> Moreover, we're going to do the full badblocks lookup anyway when we
> call ->direct_access().  If we had that information earlier we can
> avoid this fallback dance.

None of the proposed approaches looks clean to me.  I'll go along with
whatever you guys think is best.  I am in favor of wrapping up all that
duplicated code, though.

Cheers,
Jeff