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

[Qi Zheng]

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.