Re: [PATCH 25/26] xfs: rework unreferenced inode lookups

Dave Chinner
2019-10-18T20:30:10Z

On Thu, Oct 17, 2019 at 03:57:29AM -0400, Brian Foster wrote:
> On Thu, Oct 17, 2019 at 12:24:38PM +1100, Dave Chinner wrote:
> > It's not a contention issue - there's real bugs if we don't order
> > the locking correctly here.
> >
>
> Is this patch fixing real bugs in the existing code or reducing
> contention/blocking in the reclaim codepath? My understanding was the
> latter, so thus I'm trying to make sure I follow how this blocking can
> actually happen that this patch purports to address. The reasoning in my
> comment above is basically how I followed the existing code as it
> pertains to blocking in reclaim, and that is the scenario I was asking
> about...

Neither. It's a patch that simplifies and formalises the
unreferenced inode lookup alogrithm. Previous patches change the way
we isolate inodes for reclaim, opening up the opportunity to
simplify the lookup/reclaim synchronisation and remove a race
condition that that we've carried a workaround to avoid for 20+
years.

Yes, it has the added bonus of removing a potential blocking point
in reclaim, but hitting that blocking point it is pretty rare so
it's not really a reduction in anything measurable.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com