Re: [PATCH 2/7] mm: vmscan: Convert lumpy_mode into a bitmask

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Dec 01, 2010 at 10:50:29AM +0000, Mel Gorman wrote:
> On Wed, Dec 01, 2010 at 11:27:32AM +0100, Johannes Weiner wrote:
> > On Mon, Nov 22, 2010 at 03:43:50PM +0000, Mel Gorman wrote:
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> > > @@ -51,11 +51,20 @@
> > >  #define CREATE_TRACE_POINTS
> > >  #include <trace/events/vmscan.h>
> > >  
> > > -enum lumpy_mode {
> > > -	LUMPY_MODE_NONE,
> > > -	LUMPY_MODE_ASYNC,
> > > -	LUMPY_MODE_SYNC,
> > > -};
> > > +/*
> > > + * lumpy_mode determines how the inactive list is shrunk
> > > + * LUMPY_MODE_SINGLE: Reclaim only order-0 pages
> > > + * LUMPY_MODE_ASYNC:  Do not block
> > > + * LUMPY_MODE_SYNC:   Allow blocking e.g. call wait_on_page_writeback
> > > + * LUMPY_MODE_CONTIGRECLAIM: For high-order allocations, take a reference
> > > + *			page from the LRU and reclaim all pages within a
> > > + *			naturally aligned range
> > 
> > I find those names terribly undescriptive.  It also strikes me as an
> > odd set of flags.  Can't this be represented with less?
> > 
> > 	LUMPY_MODE_ENABLED
> > 	LUMPY_MODE_SYNC
> > 
> > or, after the rename,
> > 
> > 	RECLAIM_MODE_HIGHER	= 1
> > 	RECLAIM_MODE_SYNC	= 2
> > 	RECLAIM_MODE_LUMPY	= 4
> > 
> 
> My problem with that is you have to infer what the behaviour is from what the
> flags "are not" as opposed to what they are. For example, !LUMPY_MODE_SYNC
> implies LUMPY_MODE_ASYNC instead of specifying LUMPY_MODE_ASYNC.

Sounds like a boolean value to me.  And it shows: you never actually
check for RECLAIM_MODE_ASYNC in the code, you just always set it to
the opposite of RECLAIM_MODE_SYNC - the flag which is actually read.

> It also looks very odd when trying to distinguish between order-0
> standard reclaim, lumpy reclaim and reclaim/compaction.

That is true, because this is still an actual tristate.  It's probably
better to defer until lumpy reclaim is gone and there is only one flag
for higher-order reclaim left.

> > > +typedef unsigned __bitwise__ lumpy_mode;
> > 
> > lumpy_mode_t / reclaim_mode_t?
> > 
> 
> It can't hurt!

Thanks :)