linux-kernel - Re: [PATCH v3 19/21] slab: remove frozen slab checks from __slab

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Message-ID: <6b48cc0f-f006-4d4c-af76-55f86f4267e3@suse.cz>
Date: Thu, 22 Jan 2026 07:31:47 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Suren Baghdasaryan <surenb@...gle.com>
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>, Hao Li <hao.li@...ux.dev>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Uladzislau Rezki <urezki@...il.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.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 v3 19/21] slab: remove frozen slab checks from
 __slab_free()

On 1/22/26 01:54, Suren Baghdasaryan wrote:
> On Fri, Jan 16, 2026 at 2:41 PM Vlastimil Babka <vbabka@...e.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@...gle.com>
>> Signed-off-by: Vlastimil Babka <vbabka@...e.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@...gle.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
>>