Re: [PATCH v4 10/22] slab: add optimized sheaf refill from partial list

Liam R. Howlett
2026-01-27T20:05:54Z

* Vlastimil Babka <vbabka@suse.cz> [260123 01:53]:
> At this point we have sheaves enabled for all caches, but their refill
> is done via __kmem_cache_alloc_bulk() which relies on cpu (partial)
> slabs - now a redundant caching layer that we are about to remove.
>
> The refill will thus be done from slabs on the node partial list.
> Introduce new functions that can do that in an optimized way as it's
> easier than modifying the __kmem_cache_alloc_bulk() call chain.
>
> Introduce struct partial_bulk_context, a variant of struct
> partial_context that can return a list of slabs from the partial list
> with the sum of free objects in them within the requested min and max.
>
> Introduce get_partial_node_bulk() that removes the slabs from freelist
> and returns them in the list. There is a racy read of slab->counters
> so make sure the non-atomic write in __update_freelist_slow() is not
> tearing.
>
> Introduce get_freelist_nofreeze() which grabs the freelist without
> freezing the slab.
>
> Introduce alloc_from_new_slab() which can allocate multiple objects from
> a newly allocated slab where we don't need to synchronize with freeing.
> In some aspects it's similar to alloc_single_from_new_slab() but assumes
> the cache is a non-debug one so it can avoid some actions. It supports
> the allow_spin parameter, which we always set true here, but the
> followup change will reuse the function in a context where it may be
> false.
>
> Introduce __refill_objects() that uses the functions above to fill an
> array of objects. It has to handle the possibility that the slabs will
> contain more objects that were requested, due to concurrent freeing of
> objects to those slabs. When no more slabs on partial lists are
> available, it will allocate new slabs. It is intended to be only used
> in context where spinning is allowed, so add a WARN_ON_ONCE check there.
>
> Finally, switch refill_sheaf() to use __refill_objects(). Sheaves are
> only refilled from contexts that allow spinning, or even blocking.
>
> Reviewed-by: Suren Baghdasaryan <surenb@google.com>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

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

> ---
> mm/slub.c | 293 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++-----
> 1 file changed, 272 insertions(+), 21 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 22acc249f9c0..142a1099bbc1 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -248,6 +248,14 @@ struct partial_context {
> void *object;
> };
>
> +/* Structure holding parameters for get_partial_node_bulk() */
> +struct partial_bulk_context {
> + gfp_t flags;
> + unsigned int min_objects;
> + unsigned int max_objects;
> + struct list_head slabs;
> +};
> +
> static inline bool kmem_cache_debug(struct kmem_cache *s)
> {
> return kmem_cache_debug_flags(s, SLAB_DEBUG_FLAGS);
> @@ -778,7 +786,8 @@ __update_freelist_slow(struct slab *slab, struct freelist_counters *old,
> slab_lock(slab);
> if (slab->freelist == old->freelist &&
> slab->counters == old->counters) {
> - slab->freelist = new->freelist;
> + /* prevent tearing for the read in get_partial_node_bulk() */
> + WRITE_ONCE(slab->freelist, new->freelist);
> slab->counters = new->counters;
> ret = true;
> }
> @@ -2638,9 +2647,9 @@ static void free_empty_sheaf(struct kmem_cache *s, struct slab_sheaf *sheaf)
> stat(s, SHEAF_FREE);
> }
>
> -static int __kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags,
> - size_t size, void **p);
> -
> +static unsigned int
> +__refill_objects(struct kmem_cache *s, void **p, gfp_t gfp, unsigned int min,
> + unsigned int max);
>
> static int refill_sheaf(struct kmem_cache *s, struct slab_sheaf *sheaf,
> gfp_t gfp)
> @@ -2651,8 +2660,8 @@ static int refill_sheaf(struct kmem_cache *s, struct slab_sheaf *sheaf,
> if (!to_fill)
> return 0;
>
> - filled = __kmem_cache_alloc_bulk(s, gfp, to_fill,
> - &sheaf->objects[sheaf->size]);
> + filled = __refill_objects(s, &sheaf->objects[sheaf->size], gfp,
> + to_fill, to_fill);
>
> sheaf->size += filled;
>
> @@ -3518,6 +3527,57 @@ static inline void put_cpu_partial(struct kmem_cache *s, struct slab *slab,
> #endif
> static inline bool pfmemalloc_match(struct slab *slab, gfp_t gfpflags);
>
> +static bool get_partial_node_bulk(struct kmem_cache *s,
> + struct kmem_cache_node *n,
> + struct partial_bulk_context *pc)
> +{
> + struct slab *slab, *slab2;
> + unsigned int total_free = 0;
> + unsigned long flags;
> +
> + /* Racy check to avoid taking the lock unnecessarily. */
> + if (!n || data_race(!n->nr_partial))
> + return false;
> +
> + INIT_LIST_HEAD(&pc->slabs);
> +
> + spin_lock_irqsave(&n->list_lock, flags);
> +
> + list_for_each_entry_safe(slab, slab2, &n->partial, slab_list) {
> + struct freelist_counters flc;
> + unsigned int slab_free;
> +
> + if (!pfmemalloc_match(slab, pc->flags))
> + continue;
> +
> + /*
> + * determine the number of free objects in the slab racily
> + *
> + * slab_free is a lower bound due to possible subsequent
> + * concurrent freeing, so the caller may get more objects than
> + * requested and must handle that
> + */
> + flc.counters = data_race(READ_ONCE(slab->counters));
> + slab_free = flc.objects - flc.inuse;
> +
> + /* we have already min and this would get us over the max */
> + if (total_free >= pc->min_objects
> + && total_free + slab_free > pc->max_objects)
> + break;
> +
> + remove_partial(n, slab);
> +
> + list_add(&slab->slab_list, &pc->slabs);
> +
> + total_free += slab_free;
> + if (total_free >= pc->max_objects)
> + break;
> + }
> +
> + spin_unlock_irqrestore(&n->list_lock, flags);
> + return total_free > 0;
> +}
> +
> /*
> * Try to allocate a partial slab from a specific node.
> */
> @@ -4444,6 +4504,33 @@ static inline void *get_freelist(struct kmem_cache *s, struct slab *slab)
> return old.freelist;
> }
>
> +/*
> + * Get the slab's freelist and do not freeze it.
> + *
> + * Assumes the slab is isolated from node partial list and not frozen.
> + *
> + * Assumes this is performed only for caches without debugging so we
> + * don't need to worry about adding the slab to the full list.
> + */
> +static inline void *get_freelist_nofreeze(struct kmem_cache *s, struct slab *slab)
> +{
> + struct freelist_counters old, new;
> +
> + do {
> + old.freelist = slab->freelist;
> + old.counters = slab->counters;
> +
> + new.freelist = NULL;
> + new.counters = old.counters;
> + VM_WARN_ON_ONCE(new.frozen);
> +
> + new.inuse = old.objects;
> +
> + } while (!slab_update_freelist(s, slab, &old, &new, "get_freelist_nofreeze"));
> +
> + return old.freelist;
> +}
> +
> /*
> * Freeze the partial slab and return the pointer to the freelist.
> */
> @@ -4467,6 +4554,72 @@ static inline void *freeze_slab(struct kmem_cache *s, struct slab *slab)
> return old.freelist;
> }
>
> +/*
> + * If the object has been wiped upon free, make sure it's fully initialized by
> + * zeroing out freelist pointer.
> + *
> + * Note that we also wipe custom freelist pointers.
> + */
> +static __always_inline void maybe_wipe_obj_freeptr(struct kmem_cache *s,
> + void *obj)
> +{
> + if (unlikely(slab_want_init_on_free(s)) && obj &&
> + !freeptr_outside_object(s))
> + memset((void *)((char *)kasan_reset_tag(obj) + s->offset),
> + 0, sizeof(void *));
> +}
> +
> +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;
> + bool needs_add_partial;
> + unsigned long flags;
> + void *object;
> +
> + /*
> + * Are we going to put the slab on the partial list?
> + * Note slab->inuse is 0 on a new slab.
> + */
> + needs_add_partial = (slab->objects > count);
> +
> + if (!allow_spin && needs_add_partial) {
> +
> + 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 (needs_add_partial) {
> +
> + 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);
> + return allocated;
> +}
> +
> /*
> * Slow path. The lockless freelist is empty or we need to perform
> * debugging duties.
> @@ -4909,21 +5062,6 @@ static __always_inline void *__slab_alloc_node(struct kmem_cache *s,
> return object;
> }
>
> -/*
> - * If the object has been wiped upon free, make sure it's fully initialized by
> - * zeroing out freelist pointer.
> - *
> - * Note that we also wipe custom freelist pointers.
> - */
> -static __always_inline void maybe_wipe_obj_freeptr(struct kmem_cache *s,
> - void *obj)
> -{
> - if (unlikely(slab_want_init_on_free(s)) && obj &&
> - !freeptr_outside_object(s))
> - memset((void *)((char *)kasan_reset_tag(obj) + s->offset),
> - 0, sizeof(void *));
> -}
> -
> static __fastpath_inline
> struct kmem_cache *slab_pre_alloc_hook(struct kmem_cache *s, gfp_t flags)
> {
> @@ -5384,6 +5522,9 @@ static int __prefill_sheaf_pfmemalloc(struct kmem_cache *s,
> return ret;
> }
>
> +static int __kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags,
> + size_t size, void **p);
> +
> /*
> * returns a sheaf that has at least the requested size
> * when prefilling is needed, do so with given gfp flags
> @@ -7484,6 +7625,116 @@ void kmem_cache_free_bulk(struct kmem_cache *s, size_t size, void **p)
> }
> EXPORT_SYMBOL(kmem_cache_free_bulk);
>
> +static unsigned int
> +__refill_objects(struct kmem_cache *s, void **p, gfp_t gfp, unsigned int min,
> + unsigned int max)
> +{
> + struct partial_bulk_context pc;
> + struct slab *slab, *slab2;
> + unsigned int refilled = 0;
> + unsigned long flags;
> + void *object;
> + int node;
> +
> + pc.flags = gfp;
> + pc.min_objects = min;
> + pc.max_objects = max;
> +
> + node = numa_mem_id();
> +
> + if (WARN_ON_ONCE(!gfpflags_allow_spinning(gfp)))
> + return 0;
> +
> + /* TODO: consider also other nodes? */
> + if (!get_partial_node_bulk(s, get_node(s, node), &pc))
> + goto new_slab;
> +
> + list_for_each_entry_safe(slab, slab2, &pc.slabs, slab_list) {
> +
> + list_del(&slab->slab_list);
> +
> + object = get_freelist_nofreeze(s, slab);
> +
> + while (object && refilled < max) {
> + p[refilled] = object;
> + object = get_freepointer(s, object);
> + maybe_wipe_obj_freeptr(s, p[refilled]);
> +
> + refilled++;
> + }
> +
> + /*
> + * Freelist had more objects than we can accommodate, we need to
> + * free them back. We can treat it like a detached freelist, just
> + * need to find the tail object.
> + */
> + if (unlikely(object)) {
> + void *head = object;
> + void *tail;
> + int cnt = 0;
> +
> + do {
> + tail = object;
> + cnt++;
> + object = get_freepointer(s, object);
> + } while (object);
> + do_slab_free(s, slab, head, tail, cnt, _RET_IP_);
> + }
> +
> + if (refilled >= max)
> + break;
> + }
> +
> + if (unlikely(!list_empty(&pc.slabs))) {
> + struct kmem_cache_node *n = get_node(s, node);
> +
> + spin_lock_irqsave(&n->list_lock, flags);
> +
> + list_for_each_entry_safe(slab, slab2, &pc.slabs, slab_list) {
> +
> + if (unlikely(!slab->inuse && n->nr_partial >= s->min_partial))
> + continue;
> +
> + list_del(&slab->slab_list);
> + add_partial(n, slab, DEACTIVATE_TO_HEAD);
> + }
> +
> + spin_unlock_irqrestore(&n->list_lock, flags);
> +
> + /* any slabs left are completely free and for discard */
> + list_for_each_entry_safe(slab, slab2, &pc.slabs, slab_list) {
> +
> + list_del(&slab->slab_list);
> + discard_slab(s, slab);
> + }
> + }
> +
> +
> + if (likely(refilled >= min))
> + goto out;
> +
> +new_slab:
> +
> + slab = new_slab(s, pc.flags, node);
> + if (!slab)
> + goto out;
> +
> + stat(s, ALLOC_SLAB);
> +
> + /*
> + * TODO: possible optimization - if we know we will consume the whole
> + * slab we might skip creating the freelist?
> + */
> + refilled += alloc_from_new_slab(s, slab, p + refilled, max - refilled,
> + /* allow_spin = */ true);
> +
> + if (refilled < min)
> + goto new_slab;
> +out:
> +
> + return refilled;
> +}
> +
> static inline
> int __kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
> void **p)
>
> --
> 2.52.0
>