Re: [PATCH v3 07/21] slab: make percpu sheaves compatible with kmalloc_nolock()/kfree_nolock()

Vlastimil Babka
2026-01-19T10:23:07Z

On 1/19/26 11:09, Vlastimil Babka wrote:
> On 1/19/26 05:31, Harry Yoo wrote:
>> On Fri, Jan 16, 2026 at 03:40:27PM +0100, Vlastimil Babka wrote:
>>> 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().
>>>
>>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
>>> ---
>>
>> Looks good to me,
>> Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
>
> Thanks.
>
>>
>> with a nit below.
>>
>>> mm/slub.c | 79 ++++++++++++++++++++++++++++++++++++++++++++-------------------
>>> 1 file changed, 56 insertions(+), 23 deletions(-)
>>>
>>> diff --git a/mm/slub.c b/mm/slub.c
>>> index 706cb6398f05..b385247c219f 100644
>>> --- a/mm/slub.c
>>> +++ b/mm/slub.c
>>> @@ -6703,7 +6735,7 @@ 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;
>>> }
>>>
>>> @@ -6964,7 +6996,8 @@ 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);
>>> - do_slab_free(s, slab, x, x, 0, _RET_IP_);
>>> + if (!free_to_pcs(s, x, false))
>>> + do_slab_free(s, slab, x, x, 0, _RET_IP_);
>>> }
>>
>> nit: Maybe it's not that common but should we bypass sheaves if
>> it's from remote NUMA node just like slab_free()?
>
> Right, will do.

However that means sheaves will help less with the defer_free() avoidance
here. It becomes more obvious after "slab: remove the do_slab_free()
fastpath". All remote object frees will be deferred. Guess we can revisit
later if we see there are too many and have no better solution...

>>> EXPORT_SYMBOL_GPL(kfree_nolock);
>>>
>>> @@ -7516,7 +7549,7 @@ 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) { > /*
>>>
>>
>