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SubjectRe: [PATCH 18/22] Use allocation flags as an index to the zone watermark
On Wed, Apr 22, 2009 at 01:06:07PM -0700, David Rientjes wrote:
> On Wed, 22 Apr 2009, Mel Gorman wrote:
>
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index b174f2c..6030f49 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1154,10 +1154,15 @@ failed:
> > return NULL;
> > }
> >
> > -#define ALLOC_NO_WATERMARKS 0x01 /* don't check watermarks at all */
> > -#define ALLOC_WMARK_MIN 0x02 /* use pages_min watermark */
> > -#define ALLOC_WMARK_LOW 0x04 /* use pages_low watermark */
> > -#define ALLOC_WMARK_HIGH 0x08 /* use pages_high watermark */
> > +/* The WMARK bits are used as an index zone->pages_mark */
> > +#define ALLOC_WMARK_MIN 0x00 /* use pages_min watermark */
> > +#define ALLOC_WMARK_LOW 0x01 /* use pages_low watermark */
> > +#define ALLOC_WMARK_HIGH 0x02 /* use pages_high watermark */
> > +#define ALLOC_NO_WATERMARKS 0x04 /* don't check watermarks at all */
> > +
> > +/* Mask to get the watermark bits */
> > +#define ALLOC_WMARK_MASK (ALLOC_NO_WATERMARKS-1)
> > +
> > #define ALLOC_HARDER 0x10 /* try to alloc harder */
> > #define ALLOC_HIGH 0x20 /* __GFP_HIGH set */
> > #define ALLOC_CPUSET 0x40 /* check for correct cpuset */
>
> The watermark flags should probably be members of an anonymous enum since
> they're being used as an index into an array. If another watermark were
> ever to be added it would require a value of 0x03, for instance.
>
> enum {
> ALLOC_WMARK_MIN,
> ALLOC_WMARK_LOW,
> ALLOC_WMARK_HIGH,
>
> ALLOC_WMARK_MASK = 0xf /* no more than 16 possible watermarks */
> };
>
> This eliminates ALLOC_NO_WATERMARKS and the caller that uses it would
> simply pass 0.
>

I'm missing something here. If ALLOC_NO_WATERMARKS was defined as zero
then thing like this break.

if (likely(!(gfp_mask & __GFP_NOMEMALLOC))) {
if (!in_interrupt() &&
((p->flags & PF_MEMALLOC) ||
unlikely(test_thread_flag(TIF_MEMDIE))))
alloc_flags |= ALLOC_NO_WATERMARKS;
}

Also, the ALLOC_HARDER and other alloc flags need to be redefined for
ALLOC_WMARK_MASK == 0xf. I know what you are getting at but it's a bit more
involved than you're making out and I'm not seeing an advantage.

> > @@ -1445,12 +1450,7 @@ zonelist_scan:
> >
> > if (!(alloc_flags & ALLOC_NO_WATERMARKS)) {
>
> This would become
>
> if (alloc_flags & ALLOC_WMARK_MASK)
>

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


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