Re: [PATCH 2/8] Fragmentation Avoidance V17: 002_usemap

[Mel Gorman]

On Thu, 13 Oct 2005, Dave Hansen wrote:

> > > > + * RCLM_SHIFT is the number of bits that a gfp_mask has to be shifted right
> > > > + * to have just the __GFP_USER and __GFP_KERNRCLM bits. The static check is
> > > > + * made afterwards in case the GFP flags are not updated without updating
> > > > + * this number
> > > > + */
> > > > +#define RCLM_SHIFT 19
> > > > +#if (__GFP_USER >> RCLM_SHIFT) != RCLM_USER
> > > > +#error __GFP_USER not mapping to RCLM_USER
> > > > +#endif
> > > > +#if (__GFP_KERNRCLM >> RCLM_SHIFT) != RCLM_KERN
> > > > +#error __GFP_KERNRCLM not mapping to RCLM_KERN
> > > > +#endif
> > >
> > > Should this really be in page_alloc.c, or should it be close to the
> > > RCLM_* definitions?
> >
> > I can't test it right now, but I think the reason it is here is because
> > RCLM_* and __GFP_* are in different headers that are not aware of each
> > other. This is the place a static compile-time check can be made.
>
> Well, they're pretty intricately linked, so maybe they should go in the
> same header, no?
>

I looked into this more and I did have a good reason for putting them in
different headers. The __GFP_* flags have to be defined with the other GFP
flags, it just does not make sense otherwise. The RCLM_* flags must be
with the definition of struct zone * because there determine the number of
free lists that exist in the zone. There is no obvious way to have the
RCLM_* and __GFP_* flags in the same place unless mmzone.h includes gfp.h
which I seriously doubt we want.

The value of RCLM_SHIFT depends on both __GFP_* and RCLM_* so it needs to
be defined in a place that can see both gfp.h and mmzone.h . As
page_alloc.c is the only user of RCLM_SHIFT, it made sense to define it
there.

Is there a clearer way of doing this?
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