Unable to understand a piece of code

[Máté Eckl]

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++) \
> if ((__n = get_node(__s, __node)))
> 
> As we see for loop is based on node.

Hi,

This sentence is almost correct. The for loop is *not based* on node but *uses*
the node variable. If you check, __node gets initialised to 0 at the beginning of
the for loop. So whatever you use that macro with will be initialised to 0 at
the beginning of the for loop.

Bests,
Máté

> 
> Thanks in advance.
> 
> Regards,
> Amit Kumar