I have some doubts about the loop to find the gfporder of a cache. For
the code below, its main purpose is to find a gfporder value that can
make the internal fragmentation less that 1/8 of the total slab size.
It is done by increase gfporder for low number to high(possibly 0 to
MAX_GFP_ORDER). But why increase the gfporder(or slab size) can
decrease the internal fragmentation?)

A simple example, suppose the slab management stuff is kept off-slab,
if the gfporder is zero, and the object size in slab is 1000, the
wasted space is 4096 mod 1000 = 96, but with 4096 * 2(increase
gfporder by 1), the space is 8192 mod 1000 = 192, 192 > 96.

Is it right?

By the way, is the first time gfporder is 0? Who initialized it in
cache_cache?

/* Cal size (in pages) of slabs, and the num of objs per slab.
* This could be made much more intelligent. For now, try to avoid
* using high page-orders for slabs. When the gfp() funcs are more
* friendly towards high-order requests, this should be changed.
*/
do {
unsigned int break_flag = 0;
cal_wastage:
kmem_cache_estimate(cachep->gfporder, size, flags,
&left_over, &cachep->num);
if (break_flag)
break;
if (cachep->gfporder >= MAX_GFP_ORDER)
break;
if (!cachep->num)
goto next;
if (flags & CFLGS_OFF_SLAB && cachep->num > offslab_limit) {
/* Oops, this num of objs will cause problems. */
cachep->gfporder--;
break_flag++;
goto cal_wastage;
}

/*
* Large num of objs is good, but v. large slabs are currently
* bad for the gfp()s.
*/
if (cachep->gfporder >= slab_break_gfp_order)
break;

if ((left_over*8) <= (PAGE_SIZE<gfporder))
break; /* Acceptable internal fragmentation. */
next:
cachep->gfporder++;
} while (1);