Re: [PATCH 07/11] xfs: superblock scrub should use uncached buffers

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Fri, Apr 20, 2018 at 10:07:16AM +1000, Dave Chinner wrote:
> On Thu, Apr 19, 2018 at 02:57:23PM -0400, Brian Foster wrote:
> > On Thu, Apr 19, 2018 at 10:25:48AM -0700, Darrick J. Wong wrote:
> > > On Thu, Apr 19, 2018 at 08:55:47AM -0400, Brian Foster wrote:
> > > > On Tue, Apr 17, 2018 at 07:40:20PM -0700, Darrick J. Wong wrote:
> > > > > From: Darrick J. Wong <darrick.wong@oracle.com>
> > > > > 
> > > > > We've never cached buffers when reading or writing superblocks, so we
> > > > > need to change scrub to do likewise or risk screwing up the uncached sb
> > > > > buffer usage everywhere else.
> > > > > 
> > > > > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > > > > ---
> > > > 
> > > > Hmm, so this goes back to this[1] thread, right?
> > > 
> > > Yep.
> > > 
> > > > IIRC, we wanted to end up with an lru-bypassing uncached buffer lookup
> > > > mechanism to provide uncached behavior for resource-saving purposes
> > > > but without introducing serialization issues between multiple users of
> > > > uncached buffers.
> > > 
> > > Right.
> > > 
> > > > On a quick look back, growfs currently uses cached buffers for secondary
> > > > superblocks and the associated patch was looking to change that to
> > > > something like the above. Don't we have the same requirement here (since
> > > > growfs currently still uses cached buffers)?
> > > 
> > > Correct, this patch is contingent on landing Dave's "tableise growfs"
> > > series that converts growfs to use uncached buffers for writing out the
> > > new secondary supers.  If I manage to land Dave's growfs thing before
> > > repair then I'll keep this; if repair goes first I'll defer it to that
> > > series.
> > > 
> > 
> > Ok, though I'm not sure I see the need for the higher level series
> > dependency. Don't one of these patches just need to fold in a helper[1]
> > to set the lru reference appropriately and start to use it?
> 
> Yes, that would work. :P

Ok, I'll fold that into my patch and I'll fix things up depending on who
wins the race.

I changed the ASSERT to trip on agno == 0, btw.

--D

> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com
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