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SubjectRe: [External] Re: [PATCH v1] include/linux/gfp.h: getting rid of GFP_ZONE_TABLE/BAD
On Fri, May 11, 2018 at 03:24:34AM +0000, Huaisheng HS1 Ye wrote:
> > From: owner-linux-mm@kvack.org [mailto:owner-linux-mm@kvack.org] On Behalf Of Matthew
> > Wilcox
> > On Fri, May 11, 2018 at 12:10:25AM +0800, Huaisheng Ye wrote:
> > > -#define __GFP_DMA ((__force gfp_t)___GFP_DMA)
> > > -#define __GFP_HIGHMEM ((__force gfp_t)___GFP_HIGHMEM)
> > > -#define __GFP_DMA32 ((__force gfp_t)___GFP_DMA32)
> > > +#define __GFP_DMA ((__force gfp_t)OPT_ZONE_DMA ^ ZONE_NORMAL)
> > > +#define __GFP_HIGHMEM ((__force gfp_t)ZONE_MOVABLE ^ ZONE_NORMAL)
> > > +#define __GFP_DMA32 ((__force gfp_t)OPT_ZONE_DMA32 ^ ZONE_NORMAL)
> >
> > No, you've made gfp_zone even more complex than it already is.
> > If you can't use OPT_ZONE_HIGHMEM here, then this is a waste of time.
> >
> Dear Matthew,
>
> The reason why I don't use OPT_ZONE_HIGHMEM for __GFP_HIGHMEM directly is that, for x86_64 platform there is no CONFIG_HIGHMEM, so OPT_ZONE_HIGHMEM shall always be equal to ZONE_NORMAL.

Right. On 64-bit platforms, if somebody asks for HIGHMEM, they should
get NORMAL pages.

> For gfp_zone it is impossible to distinguish the meaning of lowest 3 bits in flags. How can gfp_zone to understand it comes from OPT_ZONE_HIGHMEM or ZONE_NORMAL?
> And the most pained thing is that, if __GFP_HIGHMEM with movable flag enabled, it means that ZONE_MOVABLE shall be returned.
> That is different from ZONE_DMA, ZONE_DMA32 and ZONE_NORMAL.

The point of this exercise is to actually encode the zone number in
the bottom bits of the GFP flags instead of something which has to be
interpreted into a zone number. When somebody sets __GFP_MOVABLE, they
should also be setting ZONE_MOVABLE:

-#define __GFP_MOVABLE ((__force gfp_t)___GFP_MOVABLE) /* ZONE_MOVABLE allowed */
+#define __GFP_MOVABLE ((__force gfp_t)(___GFP_MOVABLE | (ZONE_MOVABLE ^ ZONE_NORMAL)))

One thing that does need to change is:

-#define GFP_HIGHUSER_MOVABLE (GFP_HIGHUSER | __GFP_MOVABLE)
+#define GFP_HIGHUSER_MOVABLE (GFP_USER | __GFP_MOVABLE)

otherwise we'll be OR'ing ZONE_MOVABLE and ZONE_HIGHMEM together.

> I was thinking...
> Whether it is possible to use other judgement condition to decide OPT_ZONE_HIGHMEM or ZONE_MOVABLE shall be returned from gfp_zone.
>
> Sincerely,
> Huaisheng Ye
>
>
> > > static inline enum zone_type gfp_zone(gfp_t flags)
> > > {
> > > enum zone_type z;
> > > - int bit = (__force int) (flags & GFP_ZONEMASK);
> > > + z = ((__force unsigned int)flags & ___GFP_ZONE_MASK) ^ ZONE_NORMAL;
> > > +
> > > + if (z > OPT_ZONE_HIGHMEM)
> > > + z = OPT_ZONE_HIGHMEM +
> > > + !!((__force unsigned int)flags & ___GFP_MOVABLE);
> > >
> > > - z = (GFP_ZONE_TABLE >> (bit * GFP_ZONES_SHIFT)) &
> > > - ((1 << GFP_ZONES_SHIFT) - 1);
> > > - VM_BUG_ON((GFP_ZONE_BAD >> bit) & 1);
> > > + VM_BUG_ON(z > ZONE_MOVABLE);
> > > return z;
> > > }
>

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