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

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Date:   Mon, 1 May 2017 09:38:27 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Kani, Toshimitsu" <toshi.kani@....com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Verma, Vishal L" <vishal.l.verma@...el.com>
Subject: Re: [PATCH] libnvdimm: rework region badblocks clearing

On Mon, May 1, 2017 at 9:20 AM, Kani, Toshimitsu <toshi.kani@....com> wrote:
> On Mon, 2017-05-01 at 09:16 -0700, Dan Williams wrote:
>> On Mon, May 1, 2017 at 9:12 AM, Kani, Toshimitsu <toshi.kani@....com>
>> wrote:
>> > On Mon, 2017-05-01 at 08:52 -0700, Dan Williams wrote:
>> > > On Mon, May 1, 2017 at 8:43 AM, Dan Williams <dan.j.williams@...e
>> > > l.co
>> > > m> wrote:
>> > > > On Mon, May 1, 2017 at 8:34 AM, Kani, Toshimitsu <toshi.kani@hp
>> > > > e.co
>> > > > m> wrote:
>> > > > > On Sun, 2017-04-30 at 05:39 -0700, Dan Williams wrote:
>> >
>> >  :
>> > > > >
>> > > > > 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.