Re: [PATCH 4/4] memcg: multi objcg charge support

[Harry Yoo]

On 5/21/26 10:05 AM, Shakeel Butt wrote:
> On Wed, May 20, 2026 at 06:35:30PM +0900, Harry Yoo wrote:
> > > @@ -3350,19 +3405,45 @@ static void __refill_obj_stock(struct obj_cgroup *objcg,
> > >    		goto out;
> > >    	}
> > > -	stock_nr_bytes = stock->nr_bytes;
> > > -	if (READ_ONCE(stock->cached_objcg) != objcg) { /* reset if necessary */
> > > -		drain_obj_stock(stock);
> > > +	for (i = 0; i < NR_OBJ_STOCK; ++i) {
> > > +		struct obj_cgroup *cached = READ_ONCE(stock->cached[i]);
> > > +
> > > +		if (!cached) {
> > > +			if (empty_slot == -1)
> > > +				empty_slot = i;
> > > +			continue;
> > > +		}
> > > +		if (cached == objcg) {
> > > +			slot = i;
> > > +			break;
> > > +		}
> > > +	}
> > > +
> > > +	if (slot == -1) {
> > > +		slot = empty_slot;
> > > +		if (slot == -1) {
> > > +			slot = get_random_u32_below(NR_OBJ_STOCK);
> >
> > It would break kmalloc_nolock() because _get_random_bytes() uses a spinlock.
> > perhaps prandom_u32_state() should be sufficient in this case.

s/spinlock/local_lock/

> > Is there a reason why it uses random eviction, unlike multi-memcg percpu
> > charge cache?
>
> Oh I didn't know and actually we are already using get_random_u32_below() in
> refill_stock(). So, it need fixing as well. That would be a separate patch.

Ouch, I see.
> I will explore prandom_u32_state().

Thanks!

FYI, SLUB had a similar issue that was recently fixed:
commit a1e244a9f1778 ("mm/slab: use prandom if !allow_spin").

It uses prandom if spinning is not allowed when shuffling slab freelist.

--
Cheers,
Harry / Hyeonggon