Re: [PATCH v3 1/2] xfs: set a mount flag when perag reservation is active

["Darrick J. Wong" <djwong@kernel.org>]

On Fri, Mar 19, 2021 at 12:43:03PM +1100, Dave Chinner wrote:
> On Fri, Mar 19, 2021 at 12:05:06PM +1100, Dave Chinner wrote:
> > On Thu, Mar 18, 2021 at 03:19:01PM -0700, Darrick J. Wong wrote:
> > > TBH I think the COW recovery and the AG block reservation pieces are
> > > prime candidates for throwing at an xfs_pwork workqueue so we can
> > > perform those scans in parallel.
> > 
> > As I mentioned on #xfs, I think we only need to do the AG read if we
> > are near enospc. i.e. we can take the entire reservation at mount
> > time (which is fixed per-ag) and only take away the used from the
> > reservation (i.e. return to the free space pool) when we actually
> > access the AGF/AGI the first time. Or when we get a ENOSPC
> > event, which might occur when we try to take the fixed reservation
> > at mount time...
> 
> Which leaves the question about when we need to actually do the
> accounting needed to fix the bug Brian is trying to fix. Can that be
> delayed until we read the AGFs or have an ENOSPC event occur? Or
> maybe some other "we are near ENOSPC and haven't read all AGFs yet"
> threshold/trigger?

Or just load them in the background and let mount() return to userspace?

> If that's the case, then I'm happy to have this patchset proceed as
> it stands under the understanding that there will be follow up to
> make the clean, lots of space free mount case avoid reading the the
> AG headers.
> 
> If it can't be made constrained, then I think we probably need to
> come up with a different approach that doesn't require reading every
> AG header on every mount...
> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com