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

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Message-ID: <1460746909.4597.7.camel@hpe.com>
Date:	Fri, 15 Apr 2016 13:01:49 -0600
From:	Toshi Kani <toshi.kani@....com>
To:	Dan Williams <dan.j.williams@...el.com>,
	Jeff Moyer <jmoyer@...hat.com>
Cc:	"axboe@...com" <axboe@...com>, "jack@...e.cz" <jack@...e.cz>,
	"david@...morbit.com" <david@...morbit.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xfs@....sgi.com" <xfs@....sgi.com>,
	"hch@...radead.org" <hch@...radead.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
	"viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
	"linux-nvdimm@...1.01.org" <linux-nvdimm@...1.01.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-ext4@...r.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

On Fri, 2016-04-15 at 11:17 -0700, Dan Williams wrote:
> 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?
> 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.

A system running with DAX may have active data set in NVDIMM lager than RAM
size.  In this case, falling back to non-DAX will allocate page cache for
the data, which will saturate the system with memory pressure.

Thanks,
-Toshi  

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