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Message-ID: <sofeahffu5jj5xbre422lelbisfclwdul2i42j7odth3j4yzil@nyxfavdhwmuz>
Date: Thu, 15 Jan 2026 22:09:17 +0800
From: Hao Li <hao.li@...ux.dev>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Harry Yoo <harry.yoo@...cle.com>, Petr Tesarik <ptesarik@...e.com>,
Christoph Lameter <cl@...two.org>, David Rientjes <rientjes@...gle.com>,
Roman Gushchin <roman.gushchin@...ux.dev>, Andrew Morton <akpm@...ux-foundation.org>,
Uladzislau Rezki <urezki@...il.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>,
Suren Baghdasaryan <surenb@...gle.com>, Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Alexei Starovoitov <ast@...nel.org>, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
linux-rt-devel@...ts.linux.dev, bpf@...r.kernel.org, kasan-dev@...glegroups.com
Subject: Re: [PATCH RFC v2 12/20] slab: remove defer_deactivate_slab()
On Mon, Jan 12, 2026 at 04:17:06PM +0100, Vlastimil Babka wrote:
> There are no more cpu slabs so we don't need their deferred
> deactivation. The function is now only used from places where we
> allocate a new slab but then can't spin on node list_lock to put it on
> the partial list. Instead of the deferred action we can free it directly
> via __free_slab(), we just need to tell it to use _nolock() freeing of
> the underlying pages and take care of the accounting.
>
> Since free_frozen_pages_nolock() variant does not yet exist for code
> outside of the page allocator, create it as a trivial wrapper for
> __free_frozen_pages(..., FPI_TRYLOCK).
>
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> ---
> mm/internal.h | 1 +
> mm/page_alloc.c | 5 +++++
> mm/slab.h | 8 +-------
> mm/slub.c | 51 ++++++++++++++++-----------------------------------
> 4 files changed, 23 insertions(+), 42 deletions(-)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index e430da900430..1f44ccb4badf 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -846,6 +846,7 @@ static inline struct page *alloc_frozen_pages_noprof(gfp_t gfp, unsigned int ord
> struct page *alloc_frozen_pages_nolock_noprof(gfp_t gfp_flags, int nid, unsigned int order);
> #define alloc_frozen_pages_nolock(...) \
> alloc_hooks(alloc_frozen_pages_nolock_noprof(__VA_ARGS__))
> +void free_frozen_pages_nolock(struct page *page, unsigned int order);
>
> extern void zone_pcp_reset(struct zone *zone);
> extern void zone_pcp_disable(struct zone *zone);
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 822e05f1a964..8a288ecfdd93 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2981,6 +2981,11 @@ void free_frozen_pages(struct page *page, unsigned int order)
> __free_frozen_pages(page, order, FPI_NONE);
> }
>
> +void free_frozen_pages_nolock(struct page *page, unsigned int order)
> +{
> + __free_frozen_pages(page, order, FPI_TRYLOCK);
> +}
> +
> /*
> * Free a batch of folios
> */
> diff --git a/mm/slab.h b/mm/slab.h
> index e77260720994..4efec41b6445 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -71,13 +71,7 @@ struct slab {
> struct kmem_cache *slab_cache;
> union {
> struct {
> - union {
> - struct list_head slab_list;
> - struct { /* For deferred deactivate_slab() */
> - struct llist_node llnode;
> - void *flush_freelist;
> - };
> - };
> + struct list_head slab_list;
> /* Double-word boundary */
> struct freelist_counters;
> };
> diff --git a/mm/slub.c b/mm/slub.c
> index 522a7e671a26..0effeb3b9552 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3248,7 +3248,7 @@ static struct slab *new_slab(struct kmem_cache *s, gfp_t flags, int node)
> flags & (GFP_RECLAIM_MASK | GFP_CONSTRAINT_MASK), node);
> }
>
> -static void __free_slab(struct kmem_cache *s, struct slab *slab)
> +static void __free_slab(struct kmem_cache *s, struct slab *slab, bool allow_spin)
> {
> struct page *page = slab_page(slab);
> int order = compound_order(page);
> @@ -3262,11 +3262,20 @@ static void __free_slab(struct kmem_cache *s, struct slab *slab)
> free_frozen_pages(page, order);
Here we missed using the newly added allow_spin.
It should call free_frozen_pages_nolock() when !allow_spin.
--
Thanks,
Hao
> }
>
> +static void free_new_slab_nolock(struct kmem_cache *s, struct slab *slab)
> +{
> + /*
> + * Since it was just allocated, we can skip the actions in
> + * discard_slab() and free_slab().
> + */
> + __free_slab(s, slab, false);
> +}
> +
> static void rcu_free_slab(struct rcu_head *h)
> {
> struct slab *slab = container_of(h, struct slab, rcu_head);
>
> - __free_slab(slab->slab_cache, slab);
> + __free_slab(slab->slab_cache, slab, true);
> }
>
> static void free_slab(struct kmem_cache *s, struct slab *slab)
> @@ -3282,7 +3291,7 @@ static void free_slab(struct kmem_cache *s, struct slab *slab)
> if (unlikely(s->flags & SLAB_TYPESAFE_BY_RCU))
> call_rcu(&slab->rcu_head, rcu_free_slab);
> else
> - __free_slab(s, slab);
> + __free_slab(s, slab, true);
> }
>
> static void discard_slab(struct kmem_cache *s, struct slab *slab)
> @@ -3375,8 +3384,6 @@ static void *alloc_single_from_partial(struct kmem_cache *s,
> return object;
> }
>
> -static void defer_deactivate_slab(struct slab *slab, void *flush_freelist);
> -
> /*
> * Called only for kmem_cache_debug() caches to allocate from a freshly
> * allocated slab. Allocate a single object instead of whole freelist
> @@ -3392,8 +3399,8 @@ static void *alloc_single_from_new_slab(struct kmem_cache *s, struct slab *slab,
> void *object;
>
> if (!allow_spin && !spin_trylock_irqsave(&n->list_lock, flags)) {
> - /* Unlucky, discard newly allocated slab */
> - defer_deactivate_slab(slab, NULL);
> + /* Unlucky, discard newly allocated slab. */
> + free_new_slab_nolock(s, slab);
> return NULL;
> }
>
> @@ -4262,7 +4269,7 @@ static unsigned int alloc_from_new_slab(struct kmem_cache *s, struct slab *slab,
>
> if (!spin_trylock_irqsave(&n->list_lock, flags)) {
> /* Unlucky, discard newly allocated slab */
> - defer_deactivate_slab(slab, NULL);
> + free_new_slab_nolock(s, slab);
> return 0;
> }
> }
> @@ -6031,7 +6038,6 @@ static void free_to_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
>
> struct defer_free {
> struct llist_head objects;
> - struct llist_head slabs;
> struct irq_work work;
> };
>
> @@ -6039,7 +6045,6 @@ static void free_deferred_objects(struct irq_work *work);
>
> static DEFINE_PER_CPU(struct defer_free, defer_free_objects) = {
> .objects = LLIST_HEAD_INIT(objects),
> - .slabs = LLIST_HEAD_INIT(slabs),
> .work = IRQ_WORK_INIT(free_deferred_objects),
> };
>
> @@ -6052,10 +6057,9 @@ static void free_deferred_objects(struct irq_work *work)
> {
> struct defer_free *df = container_of(work, struct defer_free, work);
> struct llist_head *objs = &df->objects;
> - struct llist_head *slabs = &df->slabs;
> struct llist_node *llnode, *pos, *t;
>
> - if (llist_empty(objs) && llist_empty(slabs))
> + if (llist_empty(objs))
> return;
>
> llnode = llist_del_all(objs);
> @@ -6079,16 +6083,6 @@ static void free_deferred_objects(struct irq_work *work)
>
> __slab_free(s, slab, x, x, 1, _THIS_IP_);
> }
> -
> - llnode = llist_del_all(slabs);
> - llist_for_each_safe(pos, t, llnode) {
> - struct slab *slab = container_of(pos, struct slab, llnode);
> -
> - if (slab->frozen)
> - deactivate_slab(slab->slab_cache, slab, slab->flush_freelist);
> - else
> - free_slab(slab->slab_cache, slab);
> - }
> }
>
> static void defer_free(struct kmem_cache *s, void *head)
> @@ -6102,19 +6096,6 @@ static void defer_free(struct kmem_cache *s, void *head)
> irq_work_queue(&df->work);
> }
>
> -static void defer_deactivate_slab(struct slab *slab, void *flush_freelist)
> -{
> - struct defer_free *df;
> -
> - slab->flush_freelist = flush_freelist;
> -
> - guard(preempt)();
> -
> - df = this_cpu_ptr(&defer_free_objects);
> - if (llist_add(&slab->llnode, &df->slabs))
> - irq_work_queue(&df->work);
> -}
> -
> void defer_free_barrier(void)
> {
> int cpu;
>
> --
> 2.52.0
>
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