Linux Kernel Architecture

Chapter3:MemoryManagement

Recall from Figure 3-47 that the kernel uses an interesting system to keep track of free entries: The index

of the free object that is currently under consideration is stored inslabp->free, and the index of the next

free object, is kept in the management array.

Obtaining the object that belongs to a given index is a matter of some simple pointer manipulation per-

formed inindex_to_obj.slab_bufctlis a macro that yields a pointer to thekmem_bufctlarray after

slabp.

Let us return tocache_alloc_grow. If no free object is found although all slabs have been scanned, the

cache must be enlarged usingcache_grow. This is a costly operation examined in the next section.

Growing the Cache

Figure 3-51 shows the code flow diagram forcache_grow.

Compute offset and next color

Set page pointer

Add slab to cache

cache_grow

kmem_getpages

alloc_slabmgt

alloc_pages_node

cache_init_objs

Figure 3-51: Code flow diagram forcache_grow.

The arguments ofkmem_cache_allocare passed tocache_grow. It is also possible to specify an explicit

node from which the fresh memory pages are to be supplied.

The color and offset are first calculated:

mm/slab.c

static int cache_grow(struct kmem_cache *cachep,

gfp_t flags, int nodeid, void *objp)

{

...

l3 = cachep->nodelists[nodeid];

...

offset = l3->colour_next;

l3->colour_next++;

if (l3->colour_next >= cachep->colour)

l3->colour_next = 0;

offset *= cachep->colour_off;

...

}