Re: [PATCH v3 19/21] slab: remove frozen slab checks from __slab_free()

Vlastimil Babka
2026-01-22T06:31:50Z

On 1/22/26 01:54, Suren Baghdasaryan wrote:
> On Fri, Jan 16, 2026 at 2:41 PM Vlastimil Babka <vbabka@suse.cz> wrote:
>>
>> Currently slabs are only frozen after consistency checks failed. This
>> can happen only in caches with debugging enabled, and those use
>> free_to_partial_list() for freeing. The non-debug operation of
>> __slab_free() can thus stop considering the frozen field, and we can
>> remove the FREE_FROZEN stat.
>>
>> Reviewed-by: Suren Baghdasaryan <surenb@google.com>
>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
>
> Functionally looks fine to me. Do we need to do something about the
> UAPI breakage that removal of a sysfs node might cause?

Only if someone complains. Just this week it has been reiterated by Linus:
https://lore.kernel.org/all/CAHk-%3Dwga8Qu0-OSE9VZbviq9GuqwhPhLUXeAt-S7_9%2BfMCLkKg@mail.gmail.com/

Given this is behing a config no distro enables, I think chances are good
noone will complain:

https://oracle.github.io/kconfigs/?config=UTS_RELEASE&config=SLUB_STATS

> Reviewed-by: Suren Baghdasaryan <surenb@google.com>
>
>> ---
>> mm/slub.c | 22 ++++------------------
>> 1 file changed, 4 insertions(+), 18 deletions(-)
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 476a279f1a94..7ec7049c0ca5 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -333,7 +333,6 @@ enum stat_item {
>> FREE_RCU_SHEAF_FAIL, /* Failed to free to a rcu_free sheaf */
>> FREE_FASTPATH, /* Free to cpu slab */
>> FREE_SLOWPATH, /* Freeing not to cpu slab */
>> - FREE_FROZEN, /* Freeing to frozen slab */
>> FREE_ADD_PARTIAL, /* Freeing moves slab to partial list */
>> FREE_REMOVE_PARTIAL, /* Freeing removes last object */
>> ALLOC_FROM_PARTIAL, /* Cpu slab acquired from node partial list */
>> @@ -5103,7 +5102,7 @@ static void __slab_free(struct kmem_cache *s, struct slab *slab,
>> unsigned long addr)
>>
>> {
>> - bool was_frozen, was_full;
>> + bool was_full;
>> struct freelist_counters old, new;
>> struct kmem_cache_node *n = NULL;
>> unsigned long flags;
>> @@ -5126,7 +5125,6 @@ static void __slab_free(struct kmem_cache *s, struct slab *slab,
>> old.counters = slab->counters;
>>
>> was_full = (old.freelist == NULL);
>> - was_frozen = old.frozen;
>>
>> set_freepointer(s, tail, old.freelist);
>>
>> @@ -5139,7 +5137,7 @@ static void __slab_free(struct kmem_cache *s, struct slab *slab,
>> * to (due to not being full anymore) the partial list.
>> * Unless it's frozen.
>> */
>> - if ((!new.inuse || was_full) && !was_frozen) {
>> + if (!new.inuse || was_full) {
>>
>> n = get_node(s, slab_nid(slab));
>> /*
>> @@ -5158,20 +5156,10 @@ static void __slab_free(struct kmem_cache *s, struct slab *slab,
>> } while (!slab_update_freelist(s, slab, &old, &new, "__slab_free"));
>>
>> if (likely(!n)) {
>> -
>> - if (likely(was_frozen)) {
>> - /*
>> - * The list lock was not taken therefore no list
>> - * activity can be necessary.
>> - */
>> - stat(s, FREE_FROZEN);
>> - }
>> -
>> /*
>> - * In other cases we didn't take the list_lock because the slab
>> - * was already on the partial list and will remain there.
>> + * We didn't take the list_lock because the slab was already on
>> + * the partial list and will remain there.
>> */
>> -
>> return;
>> }
>>
>> @@ -8721,7 +8709,6 @@ STAT_ATTR(FREE_RCU_SHEAF, free_rcu_sheaf);
>> STAT_ATTR(FREE_RCU_SHEAF_FAIL, free_rcu_sheaf_fail);
>> STAT_ATTR(FREE_FASTPATH, free_fastpath);
>> STAT_ATTR(FREE_SLOWPATH, free_slowpath);
>> -STAT_ATTR(FREE_FROZEN, free_frozen);
>> STAT_ATTR(FREE_ADD_PARTIAL, free_add_partial);
>> STAT_ATTR(FREE_REMOVE_PARTIAL, free_remove_partial);
>> STAT_ATTR(ALLOC_FROM_PARTIAL, alloc_from_partial);
>> @@ -8826,7 +8813,6 @@ static struct attribute *slab_attrs[] = {
>> &free_rcu_sheaf_fail_attr.attr,
>> &free_fastpath_attr.attr,
>> &free_slowpath_attr.attr,
>> - &free_frozen_attr.attr,
>> &free_add_partial_attr.attr,
>> &free_remove_partial_attr.attr,
>> &alloc_from_partial_attr.attr,
>>
>> --
>> 2.52.0
>>