Hi,

You can see the definition of struct slab, the counters uses same space
with inuse, objects and frozen because they are in a union. So when
counters gets a value, the other 3 members also get a value.

struct slab {
......snip......
                                       union {
                                               unsigned long counters;
                                               struct {
                                                       unsigned inuse:16;
                                                       unsigned objects:15;
                                                       unsigned frozen:1;
                                               };
                                       };
......snip......
}

Regards,
Xiongwei

On Sun, Nov 12, 2023 at 5:55 AM geng sun <sun.gengeration.sun@gmail.com> wrote:

Hi linux-mm

I find one odd variable “new” in slub, it was used before init.

Maybe the value in new was undefined.

And I can not understand how it work.

Could you explain this code for me?

Thanks.

Best regards.

static void __slab_free(struct kmem_cache *s, struct page *page,

void *head, void *tail, int cnt,

unsigned long addr)

{

void *prior;

int was_frozen;

struct page new;

unsigned long counters;

struct kmem_cache_node *n = NULL;

unsigned long flags;

stat(s, FREE_SLOWPATH);

if (kmem_cache_debug(s) &&

!free_debug_processing(s, page, head, tail, cnt, addr))

return;

do {

if (unlikely(n)) {

spin_unlock_irqrestore(&n->list_lock, flags);

n = NULL;

}

prior = page->freelist;

counters = page->counters;

set_freepointer(s, tail, prior);

new.counters = counters;

was_frozen = new.frozen;

new.inuse -= cnt;

if ((!new.inuse || !prior) && !was_frozen) {