Re: [PATCH] libnvdimm: rework region badblocks clearing

[Dan Williams]

On Mon, May 1, 2017 at 9:12 AM, Kani, Toshimitsu <toshi.kani@xxxxxxx> 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@xxxxxxxx
>> m> wrote:
>> > On Mon, May 1, 2017 at 8:34 AM, Kani, Toshimitsu <toshi.kani@xxxxxx
>> > 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?