Re: [RFC] Replace the watermark-related union in struct zone with a watermark[] array

Mel Gorman
2009-04-27T20:54:20Z

On Mon, Apr 27, 2009 at 01:48:47PM -0700, David Rientjes wrote:
> On Mon, 27 Apr 2009, Mel Gorman wrote:
>
> > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > index c1fa208..1ff59fd 100644
> > --- a/include/linux/mmzone.h
> > +++ b/include/linux/mmzone.h
> > @@ -163,6 +163,13 @@ static inline int is_unevictable_lru(enum lru_list l)
> > #endif
> > }
> >
> > +enum zone_watermarks {
> > + WMARK_MIN,
> > + WMARK_LOW,
> > + WMARK_HIGH,
> > + NR_WMARK
> > +};
> > +
> > struct per_cpu_pages {
> > int count; /* number of pages in the list */
> > int high; /* high watermark, emptying needed */
> > @@ -275,12 +282,9 @@ struct zone_reclaim_stat {
> >
> > struct zone {
> > /* Fields commonly accessed by the page allocator */
> > - union {
> > - struct {
> > - unsigned long pages_min, pages_low, pages_high;
> > - };
> > - unsigned long pages_mark[3];
> > - };
> > +
> > + /* zone watermarks, indexed with WMARK_LOW, WMARK_MIN and WMARK_HIGH */
> > + unsigned long watermark[NR_WMARK];
> >
> > /*
> > * We don't know if the memory that we're going to allocate will be freeable
>
> I thought the suggestion was for something like
>
> #define zone_wmark_min(z) (z->pages_mark[WMARK_MIN])
> ...

Was it the only suggestion? I thought just replacing the union with an
array would be an option as well.

The #define approach also requires setter versions like

static inline set_zone_wmark_min(struct zone *z, unsigned long val)
{
z->pages_mark[WMARK_MIN] = val;
}

and you need one of those for each watermark if you are to avoid weirdness like

zone_wmark_min(z) = val;

which looks all wrong. I felt this approach would look neater and be closer
in appearance to the code that is already there and therefore less surprising.

Would people prefer a getter/setter version?

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab