Re: [PATCH v2 5/5] dax: handle media errors in dax_do_io

[Toshi Kani]

On Fri, 2016-04-15 at 11:17 -0700, Dan Williams wrote:
> On Fri, Apr 15, 2016 at 11:06 AM, Jeff Moyer <jmoyer@xxxxxxxxxx> wrote:
> > 
> > Dan Williams <dan.j.williams@xxxxxxxxx> writes:
> >Â
> > > > > 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.ÂÂI.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)?ÂÂI'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().ÂÂIf 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. ÂIn this case, falling back to non-DAX will allocate page cache for
the data, which will saturate the system with memory pressure.

Thanks,
-Toshi Â