Re: [PATCH v4 7/8] mm: vmscan: remove shrinker_rwsem from synchronize_shrinkers()

[Qi Zheng]

Hi Kirill,

On 2023/3/9 06:39, Kirill Tkhai wrote:
> On 07.03.2023 09:56, Qi Zheng wrote:
> > Now there are no readers of shrinker_rwsem, so
> > synchronize_shrinkers() does not need to hold the
> > writer of shrinker_rwsem to wait for all running
> > shinkers to complete, synchronize_srcu() is enough.
> >
> > Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> > ---
> >   mm/vmscan.c | 8 ++------
> >   1 file changed, 2 insertions(+), 6 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 7aaf6f94ac1b..ac7ab4aa344f 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -796,15 +796,11 @@ EXPORT_SYMBOL(unregister_shrinker);
> >   /**
> >    * synchronize_shrinkers - Wait for all running shrinkers to complete.
> >    *
> > - * This is equivalent to calling unregister_shrink() and register_shrinker(),
> > - * but atomically and with less overhead. This is useful to guarantee that all
> > - * shrinker invocations have seen an update, before freeing memory, similar to
> > - * rcu.
> > + * This is useful to guarantee that all shrinker invocations have seen an
> > + * update, before freeing memory.
> >    */
> >   void synchronize_shrinkers(void)
> >   {
> > -	down_write(&shrinker_rwsem);
> > -	up_write(&shrinker_rwsem);
> >   	atomic_inc(&shrinker_srcu_generation);
> >   	synchronize_srcu(&shrinker_srcu);
> >   }
>
> Just curious, callers of synchronize_shrinkers() don't want to have parallel register_shrinker() and unregister_shrink() are completed?
> Here we only should wait for parallel shrink_slab(), correct?

I think yes.

The synchronize_shrinkers() is currently only used by TTM pool.

In TTM pool, a shrinker named "drm-ttm_pool" is registered, and
the scan_objects callback will pick an entry from its own shrinker_list:

ttm_pool_shrink
--> spin_lock(&shrinker_lock);
    pt = list_first_entry(&shrinker_list, typeof(*pt), shrinker_list);
    list_move_tail(&pt->shrinker_list, &shrinker_list);
    spin_unlock(&shrinker_lock);

These entries have been removed from shrinker_list before calling
synchronize_shrinkers():

ttm_pool_fini
--> ttm_pool_type_fini
    --> spin_lock(&shrinker_lock);
	list_del(&pt->shrinker_list);
	spin_unlock(&shrinker_lock);
    synchronize_shrinkers

So IIUC, we only need to wait for the parallel shrink_slab() here. Like
its comment says:

/* We removed the pool types from the LRU, but we need to also make sure
 * that no shrinker is concurrently freeing pages from the pool.
 */

+ CC: Christian König :)

Thanks,
Qi