Re: [RFC PATCH] mm: CONFIG_NR_ZONES_EXTENDED

On Mon, Feb 1, 2016 at 9:42 PM, Andrew Morton [off-list ref] wrote:

On Wed, 27 Jan 2016 22:19:14 -0800 Dan Williams [off-list ref] wrote:

quoted

 #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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