linux-kernel - Re: [PATCH] libnvdimm: rework region badblocks clearing

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Date:   Mon, 1 May 2017 16:45:37 +0000
From:   "Kani, Toshimitsu" <toshi.kani@....com>
To:     "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "vishal.l.verma@...el.com" <vishal.l.verma@...el.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "dave.jiang@...el.com" <dave.jiang@...el.com>
Subject: Re: [PATCH] libnvdimm: rework region badblocks clearing

On Mon, 2017-05-01 at 16:42 +0000, Verma, Vishal L wrote:
> On Mon, 2017-05-01 at 09:38 -0700, Dan Williams wrote:
> > On Mon, May 1, 2017 at 9:20 AM, Kani, Toshimitsu
> > <toshi.kani@....com>
 :
> > > > > > > > Hi Dan,
> > > > > > > > 
> > > > > > > > I was testing the change with CONFIG_DEBUG_ATOMIC_SLEEP
> > > > > > > > set this time, and hit the following BUG with
> > > > > > > > BTT.  This is a separate issue (not introduced by this
> > > > > > > > patch), but it shows that we have an issue with the DSM
> > > > > > > > call path as well.
> > > > > > > 
> > > > > > > Ah, great find, thanks! We don't see this in the unit
> > > > > > > tests because the nfit_test infrastructure takes no
> > > > > > > sleeping actions in its simulated DSM path. Outside of
> > > > > > > converting btt to use sleeping locks I'm not sure I see a
> > > > > > > path forward. I wonder how bad the performance impact of
> > > > > > > that would be? Perhaps with opportunistic spinning it
> > > > > > > won't be so bad, but I don't see another choice.
> > > > > > 
> > > > > > It's worse than that. Part of the performance optimization
> > > > > > of BTT I/O was to avoid locking altogether when we could
> > > > > > rely on a BTT lane percpu, so that would also need to be
> > > > > > removed.
> > > > > 
> > > > > I do not have a good idea either, but I'd rather disable this
> > > > > clearing in the regular BTT write path than adding sleeping
> > > > > locks to BTT. Clearing a bad block in the BTT write path is
> > > > > difficult/challenging since it allocates a new block.
> > > > 
> > > > Actually, that may make things easier. Can we teach BTT to
> > > > track error blocks and clear them before they are reassigned?
> > > 
> > > I was thinking the same after sending it.  I think we should be
> > > able to do that.
> > 
> > Ok, but we obviously can't develop something that detailed while
> > the merge window is open, so I think that means we need to revert
> > commit e88da7998d7d "Revert 'libnvdimm: band aid btt vs clear
> > poison locking'" and leave BTT I/O-error-clearing disabled for this
> > cycle and try again for 4.13.
> 
> Agreed, I'll work on something to track badblocks and clear them
> outside the IO path.

Great!  Thanks Vishal!
-Toshi