Re: [PATCH 25/43] xfs: add support for zoned space reservations

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

On Sun, Dec 15, 2024 at 06:31:35AM +0100, Christoph Hellwig wrote:
> On Fri, Dec 13, 2024 at 01:01:40PM -0800, Darrick J. Wong wrote:
> > > +#define XFS_ZR_GREEDY		(1U << 0)
> > > +#define XFS_ZR_NOWAIT		(1U << 1)
> > > +#define XFS_ZR_RESERVED		(1U << 2)
> > 
> > What do these flag values mean?  Can we put that into comments?
> 
> Sure.
> 
> > > + * For XC_FREE_RTAVAILABLE only the smaller reservation required for GC and
> > > + * block zeroing is excluded from the user capacity, while XC_FREE_RTEXTENTS
> > > + * is further restricted by at least one zone as well as the optional
> > > + * persistently reserved blocks.  This allows the allocator to run more
> > > + * smoothly by not always triggering GC.
> > 
> > Hmm, so _RTAVAILABLE really means _RTNOGC?  That makes sense.
> 
> Yes, it means block available without doing further work.
> I can't say _RTNOGC is very descriptive either, but I would not mind
> a better name if someone came up with a good one :)

Hrmm, they're rt extents that are available "now", or "for cheap"...

XC_FREE_NOW_RTEXTENTS

XC_FREE_RTEXTENTS_IMMED

XC_FREE_RTEXTENTS_CHEAP

Eh, I'm not enthusiastic about any of those.  The best I can think of
is:

	XC_FREE_RTEXTENTS_NOGC,	/* space available without gc */

> > > +		spin_unlock(&zi->zi_reservation_lock);
> > > +		schedule();
> > > +		spin_lock(&zi->zi_reservation_lock);
> > > +	}
> > > +	list_del(&reservation.entry);
> > > +	spin_unlock(&zi->zi_reservation_lock);
> > 
> > Hmm.  So if I'm understanding correctly, threads wanting to write to a
> > file try to locklessly reserve space from RTAVAILABLE.
> 
> At least if there are no waiters yet, yes.
> 
> > If they can't
> > get space because the zone is nearly full / needs gc / etc then everyone
> > gets to wait FIFO style in the reclaim_reservations list.
> 
> Yes (In a way modelled after the log grant waits).
> 
> > They can be
> > woken up from the wait if either (a) someone gives back reserved space
> > or (b) the copygc empties out this zone.
> > 
> > Or if the thread isn't willing to wait, we skip the fifo and either fail
> > up to userspace
> 
> Yes.
> 
> > or just move on to the next zone?
> 
> No other zone to move to.

<nod>

> > I think I understand the general idea, but I don't quite know when we're
> > going to use the greedy algorithm.  Later I see XFS_ZR_GREEDY gets used
> > from the buffered write path, but there doesn't seem to be an obvious
> > reason why?
> 
> Posix/Linux semantics for buffered writes require us to implement
> short writes.  That is if a single (p)write(v) syscall for say 10MB
> only find 512k of space it should write those instead of failing
> with ENOSPC.  The XFS_ZR_GREEDY implements that by backing down to
> what we can allocate (and the current implementation for that is
> a little ugly, I plan to find some time for changes to the core
> percpu_counters to improve this after the code is merged).

Ah, ok.  Can you put that in the comments defining XFS_ZR_GREEDY?

--D