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

Vlastimil Babka
2026-01-23T06:53:34Z

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>
---
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