Re: [PATCH v3 28/49] dm zoned: dynamically allocate the dm-zoned-meta shrinker

[Qi Zheng]

Hi,

On 2023/7/27 16:30, Damien Le Moal wrote:
> On 7/27/23 17:04, Qi Zheng wrote:
> > In preparation for implementing lockless slab shrink, use new APIs to
> > dynamically allocate the dm-zoned-meta shrinker, so that it can be freed
> > asynchronously using kfree_rcu(). Then it doesn't need to wait for RCU
> > read-side critical section when releasing the struct dmz_metadata.
> >
> > Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> > Reviewed-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > ---
> >   drivers/md/dm-zoned-metadata.c | 28 ++++++++++++++++------------
> >   1 file changed, 16 insertions(+), 12 deletions(-)
> >
> > diff --git a/drivers/md/dm-zoned-metadata.c b/drivers/md/dm-zoned-metadata.c
> > index 9d3cca8e3dc9..0bcb26a43578 100644
> > --- a/drivers/md/dm-zoned-metadata.c
> > +++ b/drivers/md/dm-zoned-metadata.c
> > @@ -187,7 +187,7 @@ struct dmz_metadata {
> >   	struct rb_root		mblk_rbtree;
> >   	struct list_head	mblk_lru_list;
> >   	struct list_head	mblk_dirty_list;
> > -	struct shrinker		mblk_shrinker;
> > +	struct shrinker		*mblk_shrinker;
> >   
> >   	/* Zone allocation management */
> >   	struct mutex		map_lock;
> > @@ -615,7 +615,7 @@ static unsigned long dmz_shrink_mblock_cache(struct dmz_metadata *zmd,
> >   static unsigned long dmz_mblock_shrinker_count(struct shrinker *shrink,
> >   					       struct shrink_control *sc)
> >   {
> > -	struct dmz_metadata *zmd = container_of(shrink, struct dmz_metadata, mblk_shrinker);
> > +	struct dmz_metadata *zmd = shrink->private_data;
> >   
> >   	return atomic_read(&zmd->nr_mblks);
> >   }
> > @@ -626,7 +626,7 @@ static unsigned long dmz_mblock_shrinker_count(struct shrinker *shrink,
> >   static unsigned long dmz_mblock_shrinker_scan(struct shrinker *shrink,
> >   					      struct shrink_control *sc)
> >   {
> > -	struct dmz_metadata *zmd = container_of(shrink, struct dmz_metadata, mblk_shrinker);
> > +	struct dmz_metadata *zmd = shrink->private_data;
> >   	unsigned long count;
> >   
> >   	spin_lock(&zmd->mblk_lock);
> > @@ -2936,19 +2936,23 @@ int dmz_ctr_metadata(struct dmz_dev *dev, int num_dev,
> >   	 */
> >   	zmd->min_nr_mblks = 2 + zmd->nr_map_blocks + zmd->zone_nr_bitmap_blocks * 16;
> >   	zmd->max_nr_mblks = zmd->min_nr_mblks + 512;
> > -	zmd->mblk_shrinker.count_objects = dmz_mblock_shrinker_count;
> > -	zmd->mblk_shrinker.scan_objects = dmz_mblock_shrinker_scan;
> > -	zmd->mblk_shrinker.seeks = DEFAULT_SEEKS;
> >   
> >   	/* Metadata cache shrinker */
> > -	ret = register_shrinker(&zmd->mblk_shrinker, "dm-zoned-meta:(%u:%u)",
> > -				MAJOR(dev->bdev->bd_dev),
> > -				MINOR(dev->bdev->bd_dev));
> > -	if (ret) {
> > -		dmz_zmd_err(zmd, "Register metadata cache shrinker failed");
> > +	zmd->mblk_shrinker = shrinker_alloc(0,  "dm-zoned-meta:(%u:%u)",
> > +					    MAJOR(dev->bdev->bd_dev),
> > +					    MINOR(dev->bdev->bd_dev));
> > +	if (!zmd->mblk_shrinker) {
> > +		dmz_zmd_err(zmd, "Allocate metadata cache shrinker failed");
>
> ret is not set here, so dmz_ctr_metadata() will return success. You need to add:
> 		ret = -ENOMEM;
> or something.

Indeed, will fix.

> >   		goto err;
> >   	}
> >   
> > +	zmd->mblk_shrinker->count_objects = dmz_mblock_shrinker_count;
> > +	zmd->mblk_shrinker->scan_objects = dmz_mblock_shrinker_scan;
> > +	zmd->mblk_shrinker->seeks = DEFAULT_SEEKS;
> > +	zmd->mblk_shrinker->private_data = zmd;
> > +
> > +	shrinker_register(zmd->mblk_shrinker);
>
> I fail to see how this new shrinker API is better... Why isn't there a
> shrinker_alloc_and_register() function ? That would avoid adding all this code
> all over the place as the new API call would be very similar to the current
> shrinker_register() call with static allocation.

In some registration scenarios, memory needs to be allocated in advance.
So we continue to use the previous prealloc/register_prepared()
algorithm. The shrinker_alloc_and_register() is just a helper function
that combines the two, and this increases the number of APIs that
shrinker exposes to the outside, so I choose not to add this helper.

Thanks,
Qi

> > +
> >   	dmz_zmd_info(zmd, "DM-Zoned metadata version %d", zmd->sb_version);
> >   	for (i = 0; i < zmd->nr_devs; i++)
> >   		dmz_print_dev(zmd, i);
> > @@ -2995,7 +2999,7 @@ int dmz_ctr_metadata(struct dmz_dev *dev, int num_dev,
> >    */
> >   void dmz_dtr_metadata(struct dmz_metadata *zmd)
> >   {
> > -	unregister_shrinker(&zmd->mblk_shrinker);
> > +	shrinker_free(zmd->mblk_shrinker);
> >   	dmz_cleanup_metadata(zmd);
> >   	kfree(zmd);
> >   }