Re: [RFC PATCH] mm: CONFIG_NR_ZONES_EXTENDED

[Dan Williams]

On Mon, Feb 1, 2016 at 9:42 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, 27 Jan 2016 22:19:14 -0800 Dan Williams <dan.j.williams@xxxxxxxxx> 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.
 */