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SubjectRe: [RFC PATCH] mm: CONFIG_NR_ZONES_EXTENDED
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On Mon, Feb 1, 2016 at 9:42 PM, Andrew Morton <akpm@linux-foundation.org> wrote:
> On Wed, 27 Jan 2016 22:19:14 -0800 Dan Williams <dan.j.williams@intel.com> wrote:

>> #define GFP_ZONE_TABLE ( \
>> - (ZONE_NORMAL << 0 * ZONES_SHIFT) \
>> - | (OPT_ZONE_DMA << ___GFP_DMA * ZONES_SHIFT) \
>> - | (OPT_ZONE_HIGHMEM << ___GFP_HIGHMEM * ZONES_SHIFT) \
>> - | (OPT_ZONE_DMA32 << ___GFP_DMA32 * ZONES_SHIFT) \
>> - | (ZONE_NORMAL << ___GFP_MOVABLE * ZONES_SHIFT) \
>> - | (OPT_ZONE_DMA << (___GFP_MOVABLE | ___GFP_DMA) * ZONES_SHIFT) \
>> - | (ZONE_MOVABLE << (___GFP_MOVABLE | ___GFP_HIGHMEM) * ZONES_SHIFT) \
>> - | (OPT_ZONE_DMA32 << (___GFP_MOVABLE | ___GFP_DMA32) * ZONES_SHIFT) \
>> + (ZONE_NORMAL << 0 * GFP_ZONES_SHIFT) \
>> + | (OPT_ZONE_DMA << ___GFP_DMA * GFP_ZONES_SHIFT) \
>> + | (OPT_ZONE_HIGHMEM << ___GFP_HIGHMEM * GFP_ZONES_SHIFT) \
>> + | (OPT_ZONE_DMA32 << ___GFP_DMA32 * GFP_ZONES_SHIFT) \
>> + | (ZONE_NORMAL << ___GFP_MOVABLE * GFP_ZONES_SHIFT) \
>> + | (OPT_ZONE_DMA << (___GFP_MOVABLE | ___GFP_DMA) * GFP_ZONES_SHIFT) \
>> + | (ZONE_MOVABLE << (___GFP_MOVABLE | ___GFP_HIGHMEM) * GFP_ZONES_SHIFT) \
>> + | (OPT_ZONE_DMA32 << (___GFP_MOVABLE | ___GFP_DMA32) * GFP_ZONES_SHIFT) \
>> )
>
> Geeze. Congrats on decrypting this stuff. I hope. Do you think it's
> possible to comprehensibly document it all for the next poor soul who
> ventures into it?
>

It is documented, just not included in the diff context. At least the
existing documentation was enough for me to decipher that my changes
were doing the right thing:

/*
* GFP_ZONE_TABLE is a word size bitstring that is used for looking up the
* zone to use given the lowest 4 bits of gfp_t. Entries are ZONE_SHIFT long
* and there are 16 of them to cover all possible combinations of
* __GFP_DMA, __GFP_DMA32, __GFP_MOVABLE and __GFP_HIGHMEM.
*
* The zone fallback order is MOVABLE=>HIGHMEM=>NORMAL=>DMA32=>DMA.
* But GFP_MOVABLE is not only a zone specifier but also an allocation
* policy. Therefore __GFP_MOVABLE plus another zone selector is valid.
* Only 1 bit of the lowest 3 bits (DMA,DMA32,HIGHMEM) can be set to "1".
*
* bit result
* =================
* 0x0 => NORMAL
* 0x1 => DMA or NORMAL
* 0x2 => HIGHMEM or NORMAL
* 0x3 => BAD (DMA+HIGHMEM)
* 0x4 => DMA32 or DMA or NORMAL
* 0x5 => BAD (DMA+DMA32)
* 0x6 => BAD (HIGHMEM+DMA32)
* 0x7 => BAD (HIGHMEM+DMA32+DMA)
* 0x8 => NORMAL (MOVABLE+0)
* 0x9 => DMA or NORMAL (MOVABLE+DMA)
* 0xa => MOVABLE (Movable is valid only if HIGHMEM is set too)
* 0xb => BAD (MOVABLE+HIGHMEM+DMA)
* 0xc => DMA32 (MOVABLE+DMA32)
* 0xd => BAD (MOVABLE+DMA32+DMA)
* 0xe => BAD (MOVABLE+DMA32+HIGHMEM)
* 0xf => BAD (MOVABLE+DMA32+HIGHMEM+DMA)
*
* GFP_ZONES_SHIFT must be <= 2 on 32 bit platforms.
*/

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