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Message-ID: <0f12022e-5dd2-fb1c-f018-05f8ff0303ae@bytedance.com>
Date:   Wed, 26 Jul 2023 17:33:36 +0800
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Subject: Re: [PATCH v2 28/47] bcache: dynamically allocate the md-bcache
 shrinker
Content-Language: en-US
To:     Muchun Song <muchun.song@linux.dev>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org,
        kvm@vger.kernel.org, xen-devel@lists.xenproject.org,
        linux-erofs@lists.ozlabs.org,
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        dri-devel@lists.freedesktop.org, linux-arm-msm@vger.kernel.org,
        dm-devel@redhat.com, linux-raid@vger.kernel.org,
        linux-bcache@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
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        akpm@linux-foundation.org, david@fromorbit.com, tkhai@ya.ru,
        vbabka@suse.cz, roman.gushchin@linux.dev, djwong@kernel.org,
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        yujie.liu@intel.com, gregkh@linuxfoundation.org
References: <20230724094354.90817-1-zhengqi.arch@bytedance.com>
 <20230724094354.90817-29-zhengqi.arch@bytedance.com>
 <4ee26da4-314e-0517-5d9a-31fb107368ef@linux.dev>
From:   Qi Zheng <zhengqi.arch@bytedance.com>
In-Reply-To: <4ee26da4-314e-0517-5d9a-31fb107368ef@linux.dev>
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On 2023/7/26 15:32, Muchun Song wrote:
> 
> 
> On 2023/7/24 17:43, Qi Zheng wrote:
>> In preparation for implementing lockless slab shrink, use new APIs to
>> dynamically allocate the md-bcache 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 cache_set.
>>
>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>> ---
>>   drivers/md/bcache/bcache.h |  2 +-
>>   drivers/md/bcache/btree.c  | 27 ++++++++++++++++-----------
>>   drivers/md/bcache/sysfs.c  |  3 ++-
>>   3 files changed, 19 insertions(+), 13 deletions(-)
>>
>> diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
>> index 5a79bb3c272f..c622bc50f81b 100644
>> --- a/drivers/md/bcache/bcache.h
>> +++ b/drivers/md/bcache/bcache.h
>> @@ -541,7 +541,7 @@ struct cache_set {
>>       struct bio_set        bio_split;
>>       /* For the btree cache */
>> -    struct shrinker        shrink;
>> +    struct shrinker        *shrink;
>>       /* For the btree cache and anything allocation related */
>>       struct mutex        bucket_lock;
>> diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
>> index fd121a61f17c..c176c7fc77d9 100644
>> --- a/drivers/md/bcache/btree.c
>> +++ b/drivers/md/bcache/btree.c
>> @@ -667,7 +667,7 @@ static int mca_reap(struct btree *b, unsigned int 
>> min_order, bool flush)
>>   static unsigned long bch_mca_scan(struct shrinker *shrink,
>>                     struct shrink_control *sc)
>>   {
>> -    struct cache_set *c = container_of(shrink, struct cache_set, 
>> shrink);
>> +    struct cache_set *c = shrink->private_data;
>>       struct btree *b, *t;
>>       unsigned long i, nr = sc->nr_to_scan;
>>       unsigned long freed = 0;
>> @@ -734,7 +734,7 @@ static unsigned long bch_mca_scan(struct shrinker 
>> *shrink,
>>   static unsigned long bch_mca_count(struct shrinker *shrink,
>>                      struct shrink_control *sc)
>>   {
>> -    struct cache_set *c = container_of(shrink, struct cache_set, 
>> shrink);
>> +    struct cache_set *c = shrink->private_data;
>>       if (c->shrinker_disabled)
>>           return 0;
>> @@ -752,8 +752,8 @@ void bch_btree_cache_free(struct cache_set *c)
>>       closure_init_stack(&cl);
>> -    if (c->shrink.list.next)
>> -        unregister_shrinker(&c->shrink);
>> +    if (c->shrink)
>> +        shrinker_unregister(c->shrink);
>>       mutex_lock(&c->bucket_lock);
>> @@ -828,14 +828,19 @@ int bch_btree_cache_alloc(struct cache_set *c)
>>           c->verify_data = NULL;
>>   #endif
>> -    c->shrink.count_objects = bch_mca_count;
>> -    c->shrink.scan_objects = bch_mca_scan;
>> -    c->shrink.seeks = 4;
>> -    c->shrink.batch = c->btree_pages * 2;
>> +    c->shrink = shrinker_alloc(0, "md-bcache:%pU", c->set_uuid);
>> +    if (!c->shrink) {
>> +        pr_warn("bcache: %s: could not allocate shrinker\n", __func__);
>> +        return -ENOMEM;
> 
> Seems you have cheanged the semantic of this. In the past,
> it is better to have a shrinker, but now it becomes a mandatory.
> Right? I don't know if it is acceptable. From my point of view,
> just do the cleanup, don't change any behaviour.

Oh, should return 0 here, will do.

> 
>> +    }
>> +
>> +    c->shrink->count_objects = bch_mca_count;
>> +    c->shrink->scan_objects = bch_mca_scan;
>> +    c->shrink->seeks = 4;
>> +    c->shrink->batch = c->btree_pages * 2;
>> +    c->shrink->private_data = c;
>> -    if (register_shrinker(&c->shrink, "md-bcache:%pU", c->set_uuid))
>> -        pr_warn("bcache: %s: could not register shrinker\n",
>> -                __func__);
>> +    shrinker_register(c->shrink);
>>       return 0;
>>   }
>> diff --git a/drivers/md/bcache/sysfs.c b/drivers/md/bcache/sysfs.c
>> index 0e2c1880f60b..45d8af755de6 100644
>> --- a/drivers/md/bcache/sysfs.c
>> +++ b/drivers/md/bcache/sysfs.c
>> @@ -866,7 +866,8 @@ STORE(__bch_cache_set)
>>           sc.gfp_mask = GFP_KERNEL;
>>           sc.nr_to_scan = strtoul_or_return(buf);
>> -        c->shrink.scan_objects(&c->shrink, &sc);
>> +        if (c->shrink)
>> +            c->shrink->scan_objects(c->shrink, &sc);
>>       }
>>       sysfs_strtoul_clamp(congested_read_threshold_us,
>