Re: [PATCH v3 09/21] slab: add optimized sheaf refill from partial list

Harry Yoo
2026-01-20T10:22:28Z

On Tue, Jan 20, 2026 at 05:32:37PM +0800, Hao Li wrote:
> On Tue, Jan 20, 2026 at 10:41:37AM +0900, Harry Yoo wrote:
> > On Mon, Jan 19, 2026 at 11:54:18AM +0100, Vlastimil Babka wrote:
> > > On 1/19/26 07:41, Harry Yoo wrote:
> > > > On Fri, Jan 16, 2026 at 03:40:29PM +0100, Vlastimil Babka wrote:
> > > >> /*
> > > >> * Try to allocate a partial slab from a specific node.
> > > >> */
> > > >> +static unsigned int alloc_from_new_slab(struct kmem_cache *s, struct slab *slab,
> > > >> + void **p, unsigned int count, bool allow_spin)
> > > >> +{
> > > >> + unsigned int allocated = 0;
> > > >> + struct kmem_cache_node *n;
> > > >> + unsigned long flags;
> > > >> + void *object;
> > > >> +
> > > >> + if (!allow_spin && (slab->objects - slab->inuse) > count) {
> > > >> +
> > > >> + n = get_node(s, slab_nid(slab));
> > > >> +
> > > >> + if (!spin_trylock_irqsave(&n->list_lock, flags)) {
> > > >> + /* Unlucky, discard newly allocated slab */
> > > >> + defer_deactivate_slab(slab, NULL);
> > > >> + return 0;
> > > >> + }
> > > >> + }
> > > >> +
> > > >> + object = slab->freelist;
> > > >> + while (object && allocated < count) {
> > > >> + p[allocated] = object;
> > > >> + object = get_freepointer(s, object);
> > > >> + maybe_wipe_obj_freeptr(s, p[allocated]);
> > > >> +
> > > >> + slab->inuse++;
> > > >> + allocated++;
> > > >> + }
> > > >> + slab->freelist = object;
> > > >> +
> > > >> + if (slab->freelist) {
> > > >> +
> > > >> + if (allow_spin) {
> > > >> + n = get_node(s, slab_nid(slab));
> > > >> + spin_lock_irqsave(&n->list_lock, flags);
> > > >> + }
> > > >> + add_partial(n, slab, DEACTIVATE_TO_HEAD);
> > > >> + spin_unlock_irqrestore(&n->list_lock, flags);
> > > >> + }
> > > >> +
> > > >> + inc_slabs_node(s, slab_nid(slab), slab->objects);
> > > >
> > > > Maybe add a comment explaining why inc_slabs_node() doesn't need to be
> > > > called under n->list_lock?
>
> I think this is a great observation.
>
> > >
> > > Hm, we might not even be holding it. The old code also did the inc with no
> > > comment. If anything could use one, it would be in
> > > alloc_single_from_new_slab()? But that's outside the scope here.
> >
> > Ok. Perhaps worth adding something like this later, but yeah it's outside
> > the scope here.
> >
> > diff --git a/mm/slub.c b/mm/slub.c
> > index 698c0d940f06..c5a1e47dfe16 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -1633,6 +1633,9 @@ static inline void inc_slabs_node(struct kmem_cache *s, int node, int objects)
> > {
> > struct kmem_cache_node *n = get_node(s, node);
> >
> > + if (kmem_cache_debug(s))
> > + /* slab validation may generate false errors without the lock */
> > + lockdep_assert_held(&n->list_lock);
> > atomic_long_inc(&n->nr_slabs);
> > atomic_long_add(objects, &n->total_objects);
> > }
>
> Yes. This makes sense to me.
>
> Just to double-check - I noticed that inc_slabs_node() is also called by
> early_kmem_cache_node_alloc(). Could this potentially lead to false positive
> warnings for boot-time caches when debug flags are enabled?

Good point. Perhaps the condition should be

if ((slab_state != DOWN) && kmem_cache_debug(s))

--
Cheers,
Harry / Hyeonggon