Re: [PATCH slab/for-next v4 7/8] mm/slab: introduce kfree_rcu_nolock()

[Harry Yoo]

On 7/21/26 10:09 PM, Vlastimil Babka (SUSE) wrote:
> On 7/20/26 14:44, Harry Yoo (Oracle) wrote:
>> Currently, k[v]free_rcu() cannot be called in unknown context since
>> it could lead to a deadlock when called in the middle of k[v]free_rcu().
>>
>> Make users' lives easier by introducing kfree_rcu_nolock() variant,
>> now that kfree_rcu_sheaf() is available on PREEMPT_RT and
>> __kfree_rcu_sheaf() handles unknown context.
>>
>> kfree_rcu_nolock() falls back to the kvfree_rcu batching when sheaves
>> path fails.
> 
> From the code I would say it falls back to defer_kfree_rcu(), which is an
> irq_work that does kvfree_rcu batching?

Indeed that was what I intended to say, but I messed up while
writing it :-) Will fix that.

> Otherwise the sentence would contradict the following:
>
>> In most cases, the sheaves path is expected to succeed
>> and it's unnecessary to add complexity to the existing kvfree_rcu
>> batching by teaching it how to handle unknown context.
> 
> This talks about trying to perform the batching immediately without defering.

You mean the earlier sentence "kfree_rcu_nolock() falls back to the
kvfree_rcu batching when sheaves path fails", right?

>> Since defer_kfree_rcu() can be called on caches without sheaves, move
>> deferred_work_barrier() and rcu_barrier() outside the branch in
>> kvfree_rcu_barrier_on_cache().
>>
>> Signed-off-by: Harry Yoo (Oracle) <harry@xxxxxxxxxx>
> 
> Other than that and a comment correction below, LGTM.
> 
> Reviewed-by: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>

Thanks!

>> ---
>>  include/linux/rcupdate.h | 19 +++++++++++++++++++
>>  mm/slab_common.c         | 28 ++++++++++++++++++++++++++--
>>  mm/slub.c                | 24 +++++++++++++++++++++++-
>>  3 files changed, 68 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
>> index ef5bb6981133..ce0fd401352e 100644
>> --- a/include/linux/rcupdate.h
>> +++ b/include/linux/rcupdate.h
>> @@ -1099,6 +1099,7 @@ static inline void rcu_read_unlock_migrate(void)
>>   * In mm/slab_common.c, no suitable header to include here.
>>   */
>>  void kvfree_call_rcu(struct kvfree_rcu_head *head, void *ptr);
>> +void kfree_call_rcu_nolock(struct kvfree_rcu_head *head, void *ptr);
>>  
>>  /*
>>   * The BUILD_BUG_ON() makes sure the rcu_head offset can be handled. See the
>> @@ -1124,6 +1125,24 @@ do {								\
>>  		kvfree_call_rcu(NULL, (void *) (___p));		\
>>  } while (0)
>>  
>> +/**
>> + * kfree_rcu_nolock() - a version of kfree_rcu() that can be called in any context.
>> + * @ptr: pointer to kfree for double-argument invocations.
>> + * @kvrhf: the name of the struct kvfree_rcu_head within the type of @ptr.
>> + *
>> + * Objects that are not allocated via kmalloc_nolock() are not supported.
> 
> Is this true? If __kfree_rcu_sheaf() succeeds, rcu_sheaf is either flushed
> normally (call_rcu with rcu_free_sheaf()) or via the irq_work doing the
> same. The defer_kfree_rcu() fallback ends up doing a normal
> kvfree_call_rcu(). I don't see any path that ends up doing kfree_nolock().
> Am I missing something?

I added this limitation because I thought it doesn't make sense
to support this when we don't support large kmalloc and vmalloc in
unknown context.

Perhaps too limiting than it should be though.

>> + * kfree_rcu_nolock() supports 2-arg variant only.
>> + */
>> +#define kfree_rcu_nolock(ptr, kvrhf)						\
>> +do {										\
>> +	typeof (ptr) ___p = (ptr);						\
>> +										\
>> +	if (___p) {								\
>> +		BUILD_BUG_ON(offsetof(typeof(*(ptr)), kvrhf) >= 4096);		\
>> +		kfree_call_rcu_nolock(&((___p)->kvrhf), (void *) (___p));	\
>> +	}									\
>> +} while (0)
>> +
>>  /*
>>   * Place this after a lock-acquisition primitive to guarantee that
>>   * an UNLOCK+LOCK pair acts as a full barrier.  This guarantee applies

-- 
Cheers,
Harry / Hyeonggon

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