Unable to understand a piece of code

[Tobin C. Harding]

On Mon, May 20, 2019 at 05:37:41AM +0530, Amit Kumar wrote:
> HI,
> 
> mm/slub.c: line 3973
> int __kmem_cache_shrink(struct kmem_cache *s)
> {
> int node;
> int i;
> struct kmem_cache_node *n;
> struct page *page;
> struct page *t;
> struct list_head discard;
> struct list_head promote[SHRINK_PROMOTE_MAX];
> unsigned long flags;
> int ret = 0;
> 
> flush_all(s);
> for_each_kmem_cache_node(s, node, n) {
> 
> How uninitialized variable node is being used in macro for_each_kmem_cache_node?
>
> node is a local variable with no extern and not initialized.
> 
> mm/slab.h: line 490
> #define for_each_kmem_cache_node(__s, __node, __n) \
> for (__node = 0; __node < nr_node_ids; __node++) \

This _is_ the initialization of node.  Macros are kinda funky, there
isn't much more to say about that other than perhaps read up some more
about how macros work in C and read a bunch more macros in the kernel
e.g. include/linux/list.h

Good luck,
Tobin.