Re: [PATCH v4 08/22] slab: make percpu sheaves compatible with kmalloc_nolock()/kfree_nolock()

Liam R. Howlett
2026-01-27T17:36:54Z

* Vlastimil Babka <vbabka@suse.cz> [260123 01:54]:
> Before we enable percpu sheaves for kmalloc caches, we need to make sure
> kmalloc_nolock() and kfree_nolock() will continue working properly and
> not spin when not allowed to.
>
> Percpu sheaves themselves use local_trylock() so they are already
> compatible. We just need to be careful with the barn->lock spin_lock.
> Pass a new allow_spin parameter where necessary to use
> spin_trylock_irqsave().
>
> In kmalloc_nolock_noprof() we can now attempt alloc_from_pcs() safely,
> for now it will always fail until we enable sheaves for kmalloc caches
> next. Similarly in kfree_nolock() we can attempt free_to_pcs().
>
> Reviewed-by: Suren Baghdasaryan <surenb@google.com>
> Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
> Reviewed-by: Hao Li <hao.li@linux.dev>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

I'd rather have a helper that _mayspin() or something. That way, when
we know we need to or don't want to, we avoid extra instructions. We
can also avoid passing true/false and complicating the interface when
it's not necessary.

The way it is written works as well, though.

Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>

> ---
> mm/slub.c | 82 ++++++++++++++++++++++++++++++++++++++++++++++-----------------
> 1 file changed, 60 insertions(+), 22 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 41e1bf35707c..4ca6bd944854 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2889,7 +2889,8 @@ static void pcs_destroy(struct kmem_cache *s)
> s->cpu_sheaves = NULL;
> }
>
> -static struct slab_sheaf *barn_get_empty_sheaf(struct node_barn *barn)
> +static struct slab_sheaf *barn_get_empty_sheaf(struct node_barn *barn,
> + bool allow_spin)
> {
> struct slab_sheaf *empty = NULL;
> unsigned long flags;
> @@ -2897,7 +2898,10 @@ static struct slab_sheaf *barn_get_empty_sheaf(struct node_barn *barn)
> if (!data_race(barn->nr_empty))
> return NULL;
>
> - spin_lock_irqsave(&barn->lock, flags);
> + if (likely(allow_spin))
> + spin_lock_irqsave(&barn->lock, flags);
> + else if (!spin_trylock_irqsave(&barn->lock, flags))
> + return NULL;
>
> if (likely(barn->nr_empty)) {
> empty = list_first_entry(&barn->sheaves_empty,
> @@ -2974,7 +2978,8 @@ static struct slab_sheaf *barn_get_full_or_empty_sheaf(struct node_barn *barn)
> * change.
> */
> static struct slab_sheaf *
> -barn_replace_empty_sheaf(struct node_barn *barn, struct slab_sheaf *empty)
> +barn_replace_empty_sheaf(struct node_barn *barn, struct slab_sheaf *empty,
> + bool allow_spin)
> {
> struct slab_sheaf *full = NULL;
> unsigned long flags;
> @@ -2982,7 +2987,10 @@ barn_replace_empty_sheaf(struct node_barn *barn, struct slab_sheaf *empty)
> if (!data_race(barn->nr_full))
> return NULL;
>
> - spin_lock_irqsave(&barn->lock, flags);
> + if (likely(allow_spin))
> + spin_lock_irqsave(&barn->lock, flags);
> + else if (!spin_trylock_irqsave(&barn->lock, flags))
> + return NULL;
>
> if (likely(barn->nr_full)) {
> full = list_first_entry(&barn->sheaves_full, struct slab_sheaf,
> @@ -3003,7 +3011,8 @@ barn_replace_empty_sheaf(struct node_barn *barn, struct slab_sheaf *empty)
> * barn. But if there are too many full sheaves, reject this with -E2BIG.
> */
> static struct slab_sheaf *
> -barn_replace_full_sheaf(struct node_barn *barn, struct slab_sheaf *full)
> +barn_replace_full_sheaf(struct node_barn *barn, struct slab_sheaf *full,
> + bool allow_spin)
> {
> struct slab_sheaf *empty;
> unsigned long flags;
> @@ -3014,7 +3023,10 @@ barn_replace_full_sheaf(struct node_barn *barn, struct slab_sheaf *full)
> if (!data_race(barn->nr_empty))
> return ERR_PTR(-ENOMEM);
>
> - spin_lock_irqsave(&barn->lock, flags);
> + if (likely(allow_spin))
> + spin_lock_irqsave(&barn->lock, flags);
> + else if (!spin_trylock_irqsave(&barn->lock, flags))
> + return ERR_PTR(-EBUSY);
>
> if (likely(barn->nr_empty)) {
> empty = list_first_entry(&barn->sheaves_empty, struct slab_sheaf,
> @@ -5008,7 +5020,8 @@ __pcs_replace_empty_main(struct kmem_cache *s, struct slub_percpu_sheaves *pcs,
> return NULL;
> }
>
> - full = barn_replace_empty_sheaf(barn, pcs->main);
> + full = barn_replace_empty_sheaf(barn, pcs->main,
> + gfpflags_allow_spinning(gfp));
>
> if (full) {
> stat(s, BARN_GET);
> @@ -5025,7 +5038,7 @@ __pcs_replace_empty_main(struct kmem_cache *s, struct slub_percpu_sheaves *pcs,
> empty = pcs->spare;
> pcs->spare = NULL;
> } else {
> - empty = barn_get_empty_sheaf(barn);
> + empty = barn_get_empty_sheaf(barn, true);
> }
> }
>
> @@ -5165,7 +5178,8 @@ void *alloc_from_pcs(struct kmem_cache *s, gfp_t gfp, int node)
> }
>
> static __fastpath_inline
> -unsigned int alloc_from_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
> +unsigned int alloc_from_pcs_bulk(struct kmem_cache *s, gfp_t gfp, size_t size,
> + void **p)
> {
> struct slub_percpu_sheaves *pcs;
> struct slab_sheaf *main;
> @@ -5199,7 +5213,8 @@ unsigned int alloc_from_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
> return allocated;
> }
>
> - full = barn_replace_empty_sheaf(barn, pcs->main);
> + full = barn_replace_empty_sheaf(barn, pcs->main,
> + gfpflags_allow_spinning(gfp));
>
> if (full) {
> stat(s, BARN_GET);
> @@ -5700,7 +5715,7 @@ void *kmalloc_nolock_noprof(size_t size, gfp_t gfp_flags, int node)
> gfp_t alloc_gfp = __GFP_NOWARN | __GFP_NOMEMALLOC | gfp_flags;
> struct kmem_cache *s;
> bool can_retry = true;
> - void *ret = ERR_PTR(-EBUSY);
> + void *ret;
>
> VM_WARN_ON_ONCE(gfp_flags & ~(__GFP_ACCOUNT | __GFP_ZERO |
> __GFP_NO_OBJ_EXT));
> @@ -5731,6 +5746,12 @@ void *kmalloc_nolock_noprof(size_t size, gfp_t gfp_flags, int node)
> */
> return NULL;
>
> + ret = alloc_from_pcs(s, alloc_gfp, node);
> + if (ret)
> + goto success;
> +
> + ret = ERR_PTR(-EBUSY);
> +
> /*
> * Do not call slab_alloc_node(), since trylock mode isn't
> * compatible with slab_pre_alloc_hook/should_failslab and
> @@ -5767,6 +5788,7 @@ void *kmalloc_nolock_noprof(size_t size, gfp_t gfp_flags, int node)
> ret = NULL;
> }
>
> +success:
> maybe_wipe_obj_freeptr(s, ret);
> slab_post_alloc_hook(s, NULL, alloc_gfp, 1, &ret,
> slab_want_init_on_alloc(alloc_gfp, s), size);
> @@ -6087,7 +6109,8 @@ static void __pcs_install_empty_sheaf(struct kmem_cache *s,
> * unlocked.
> */
> static struct slub_percpu_sheaves *
> -__pcs_replace_full_main(struct kmem_cache *s, struct slub_percpu_sheaves *pcs)
> +__pcs_replace_full_main(struct kmem_cache *s, struct slub_percpu_sheaves *pcs,
> + bool allow_spin)
> {
> struct slab_sheaf *empty;
> struct node_barn *barn;
> @@ -6111,7 +6134,7 @@ __pcs_replace_full_main(struct kmem_cache *s, struct slub_percpu_sheaves *pcs)
> put_fail = false;
>
> if (!pcs->spare) {
> - empty = barn_get_empty_sheaf(barn);
> + empty = barn_get_empty_sheaf(barn, allow_spin);
> if (empty) {
> pcs->spare = pcs->main;
> pcs->main = empty;
> @@ -6125,7 +6148,7 @@ __pcs_replace_full_main(struct kmem_cache *s, struct slub_percpu_sheaves *pcs)
> return pcs;
> }
>
> - empty = barn_replace_full_sheaf(barn, pcs->main);
> + empty = barn_replace_full_sheaf(barn, pcs->main, allow_spin);
>
> if (!IS_ERR(empty)) {
> stat(s, BARN_PUT);
> @@ -6133,7 +6156,8 @@ __pcs_replace_full_main(struct kmem_cache *s, struct slub_percpu_sheaves *pcs)
> return pcs;
> }
>
> - if (PTR_ERR(empty) == -E2BIG) {
> + /* sheaf_flush_unused() doesn't support !allow_spin */
> + if (PTR_ERR(empty) == -E2BIG && allow_spin) {
> /* Since we got here, spare exists and is full */
> struct slab_sheaf *to_flush = pcs->spare;
>
> @@ -6158,6 +6182,14 @@ __pcs_replace_full_main(struct kmem_cache *s, struct slub_percpu_sheaves *pcs)
> alloc_empty:
> local_unlock(&s->cpu_sheaves->lock);
>
> + /*
> + * alloc_empty_sheaf() doesn't support !allow_spin and it's
> + * easier to fall back to freeing directly without sheaves
> + * than add the support (and to sheaf_flush_unused() above)
> + */
> + if (!allow_spin)
> + return NULL;
> +
> empty = alloc_empty_sheaf(s, GFP_NOWAIT);
> if (empty)
> goto got_empty;
> @@ -6200,7 +6232,7 @@ __pcs_replace_full_main(struct kmem_cache *s, struct slub_percpu_sheaves *pcs)
> * The object is expected to have passed slab_free_hook() already.
> */
> static __fastpath_inline
> -bool free_to_pcs(struct kmem_cache *s, void *object)
> +bool free_to_pcs(struct kmem_cache *s, void *object, bool allow_spin)
> {
> struct slub_percpu_sheaves *pcs;
>
> @@ -6211,7 +6243,7 @@ bool free_to_pcs(struct kmem_cache *s, void *object)
>
> if (unlikely(pcs->main->size == s->sheaf_capacity)) {
>
> - pcs = __pcs_replace_full_main(s, pcs);
> + pcs = __pcs_replace_full_main(s, pcs, allow_spin);
> if (unlikely(!pcs))
> return false;
> }
> @@ -6333,7 +6365,7 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj)
> goto fail;
> }
>
> - empty = barn_get_empty_sheaf(barn);
> + empty = barn_get_empty_sheaf(barn, true);
>
> if (empty) {
> pcs->rcu_free = empty;
> @@ -6453,7 +6485,7 @@ static void free_to_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
> goto no_empty;
>
> if (!pcs->spare) {
> - empty = barn_get_empty_sheaf(barn);
> + empty = barn_get_empty_sheaf(barn, true);
> if (!empty)
> goto no_empty;
>
> @@ -6467,7 +6499,7 @@ static void free_to_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
> goto do_free;
> }
>
> - empty = barn_replace_full_sheaf(barn, pcs->main);
> + empty = barn_replace_full_sheaf(barn, pcs->main, true);
> if (IS_ERR(empty)) {
> stat(s, BARN_PUT_FAIL);
> goto no_empty;
> @@ -6719,7 +6751,7 @@ void slab_free(struct kmem_cache *s, struct slab *slab, void *object,
>
> if (likely(!IS_ENABLED(CONFIG_NUMA) || slab_nid(slab) == numa_mem_id())
> && likely(!slab_test_pfmemalloc(slab))) {
> - if (likely(free_to_pcs(s, object)))
> + if (likely(free_to_pcs(s, object, true)))
> return;
> }
>
> @@ -6980,6 +7012,12 @@ void kfree_nolock(const void *object)
> * since kasan quarantine takes locks and not supported from NMI.
> */
> kasan_slab_free(s, x, false, false, /* skip quarantine */true);
> +
> + if (likely(!IS_ENABLED(CONFIG_NUMA) || slab_nid(slab) == numa_mem_id())) {
> + if (likely(free_to_pcs(s, x, false)))
> + return;
> + }
> +
> do_slab_free(s, slab, x, x, 0, _RET_IP_);
> }
> EXPORT_SYMBOL_GPL(kfree_nolock);
> @@ -7532,7 +7570,7 @@ int kmem_cache_alloc_bulk_noprof(struct kmem_cache *s, gfp_t flags, size_t size,
> size--;
> }
>
> - i = alloc_from_pcs_bulk(s, size, p);
> + i = alloc_from_pcs_bulk(s, flags, size, p);
>
> if (i < size) {
> /*
>
> --
> 2.52.0
>
>