Re: [PATCH v4 45/48] mm: shrinker: make global slab shrink lockless

[Qi Zheng <zhengqi.arch@bytedance.com>]

Hi Dave,

On 2023/8/8 10:24, Dave Chinner wrote:
> On Mon, Aug 07, 2023 at 07:09:33PM +0800, Qi Zheng wrote:
>> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
>> index eb342994675a..f06225f18531 100644
>> --- a/include/linux/shrinker.h
>> +++ b/include/linux/shrinker.h
>> @@ -4,6 +4,8 @@
>>   
>>   #include <linux/atomic.h>
>>   #include <linux/types.h>
>> +#include <linux/refcount.h>
>> +#include <linux/completion.h>
>>   
>>   #define SHRINKER_UNIT_BITS	BITS_PER_LONG
>>   
>> @@ -87,6 +89,10 @@ struct shrinker {
>>   	int seeks;	/* seeks to recreate an obj */
>>   	unsigned flags;
>>   
>> +	refcount_t refcount;
>> +	struct completion done;
>> +	struct rcu_head rcu;
> 
> Documentation, please. What does the refcount protect, what does the
> completion provide, etc.

How about the following:

	/*
	 * reference count of this shrinker, holding this can guarantee
	 * that the shrinker will not be released.
	 */
	refcount_t refcount;
	/*
	 * Wait for shrinker::refcount to reach 0, that is, no shrinker
	 * is running or will run again.
	 */
	struct completion done;

> 
>> +
>>   	void *private_data;
>>   
>>   	/* These are for internal use */
>> @@ -120,6 +126,17 @@ struct shrinker *shrinker_alloc(unsigned int flags, const char *fmt, ...);
>>   void shrinker_register(struct shrinker *shrinker);
>>   void shrinker_free(struct shrinker *shrinker);
>>   
>> +static inline bool shrinker_try_get(struct shrinker *shrinker)
>> +{
>> +	return refcount_inc_not_zero(&shrinker->refcount);
>> +}
>> +
>> +static inline void shrinker_put(struct shrinker *shrinker)
>> +{
>> +	if (refcount_dec_and_test(&shrinker->refcount))
>> +		complete(&shrinker->done);
>> +}
>> +
>>   #ifdef CONFIG_SHRINKER_DEBUG
>>   extern int __printf(2, 3) shrinker_debugfs_rename(struct shrinker *shrinker,
>>   						  const char *fmt, ...);
>> diff --git a/mm/shrinker.c b/mm/shrinker.c
>> index 1911c06b8af5..d318f5621862 100644
>> --- a/mm/shrinker.c
>> +++ b/mm/shrinker.c
>> @@ -2,6 +2,7 @@
>>   #include <linux/memcontrol.h>
>>   #include <linux/rwsem.h>
>>   #include <linux/shrinker.h>
>> +#include <linux/rculist.h>
>>   #include <trace/events/vmscan.h>
>>   
>>   #include "internal.h"
>> @@ -577,33 +578,42 @@ unsigned long shrink_slab(gfp_t gfp_mask, int nid, struct mem_cgroup *memcg,
>>   	if (!mem_cgroup_disabled() && !mem_cgroup_is_root(memcg))
>>   		return shrink_slab_memcg(gfp_mask, nid, memcg, priority);
>>   
>> -	if (!down_read_trylock(&shrinker_rwsem))
>> -		goto out;
>> -
>> -	list_for_each_entry(shrinker, &shrinker_list, list) {
>> +	rcu_read_lock();
>> +	list_for_each_entry_rcu(shrinker, &shrinker_list, list) {
>>   		struct shrink_control sc = {
>>   			.gfp_mask = gfp_mask,
>>   			.nid = nid,
>>   			.memcg = memcg,
>>   		};
>>   
>> +		if (!shrinker_try_get(shrinker))
>> +			continue;
>> +
>> +		/*
>> +		 * We can safely unlock the RCU lock here since we already
>> +		 * hold the refcount of the shrinker.
>> +		 */
>> +		rcu_read_unlock();
>> +
>>   		ret = do_shrink_slab(&sc, shrinker, priority);
>>   		if (ret == SHRINK_EMPTY)
>>   			ret = 0;
>>   		freed += ret;
>> +
>>   		/*
>> -		 * Bail out if someone want to register a new shrinker to
>> -		 * prevent the registration from being stalled for long periods
>> -		 * by parallel ongoing shrinking.
>> +		 * This shrinker may be deleted from shrinker_list and freed
>> +		 * after the shrinker_put() below, but this shrinker is still
>> +		 * used for the next traversal. So it is necessary to hold the
>> +		 * RCU lock first to prevent this shrinker from being freed,
>> +		 * which also ensures that the next shrinker that is traversed
>> +		 * will not be freed (even if it is deleted from shrinker_list
>> +		 * at the same time).
>>   		 */
> 
> This needs to be moved to the head of the function, and document
> the whole list walk, get, put and completion parts of the algorithm
> that make it safe. There's more to this than "we hold a reference
> count", especially the tricky "we might see the shrinker before it
> is fully initialised" case....

How about moving these documents to before list_for_each_entry_rcu(),
and then go to the head of shrink_slab_memcg() to explain the memcg
slab shrink case.

> 
> 
> .....
>>   void shrinker_free(struct shrinker *shrinker)
>>   {
>>   	struct dentry *debugfs_entry = NULL;
>> @@ -686,9 +712,18 @@ void shrinker_free(struct shrinker *shrinker)
>>   	if (!shrinker)
>>   		return;
>>   
>> +	if (shrinker->flags & SHRINKER_REGISTERED) {
>> +		shrinker_put(shrinker);
>> +		wait_for_completion(&shrinker->done);
>> +	}
> 
> Needs a comment explaining why we need to wait here...

/*
  * Wait for all lookups of the shrinker to complete, after that, no
  * shrinker is running or will run again, then we can safely free
  * the structure where the shrinker is located, such as super_block
  * etc.
  */

>> +
>>   	down_write(&shrinker_rwsem);
>>   	if (shrinker->flags & SHRINKER_REGISTERED) {
>> -		list_del(&shrinker->list);
>> +		/*
>> +		 * Lookups on the shrinker are over and will fail in the future,
>> +		 * so we can now remove it from the lists and free it.
>> +		 */
> 
> .... rather than here after the wait has been done and provided the
> guarantee that no shrinker is running or will run again...

With the above comment, how about simplifying the comment here to the
following:

/*
  * Now we can safely remove it from the shrinker_list and free it.
  */

Thanks,
Qi

> 
> -Dave.