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Intel Mac OS X 10.15; rv:102.0) Gecko/20100101 Thunderbird/102.12.0 Subject: Re: [PATCH 02/29] mm: vmscan: introduce some helpers for dynamically allocating shrinker Content-Language: en-US To: Dave Chinner Cc: akpm@linux-foundation.org, tkhai@ya.ru, vbabka@suse.cz, roman.gushchin@linux.dev, djwong@kernel.org, brauner@kernel.org, paulmck@kernel.org, tytso@mit.edu, linux-kernel@vger.kernel.org, linux-mm@kvack.org, intel-gfx@lists.freedesktop.org, 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, linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, linux-nfs@vger.kernel.org, linux-xfs@vger.kernel.org, linux-btrfs@vger.kernel.org References: <20230622085335.77010-1-zhengqi.arch@bytedance.com> <20230622085335.77010-3-zhengqi.arch@bytedance.com> From: Qi Zheng In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-raid@vger.kernel.org Hi Dave, On 2023/6/23 14:12, Dave Chinner wrote: > On Thu, Jun 22, 2023 at 04:53:08PM +0800, Qi Zheng wrote: >> Introduce some helpers for dynamically allocating shrinker instance, >> and their uses are as follows: >> >> 1. shrinker_alloc_and_init() >> >> Used to allocate and initialize a shrinker instance, the priv_data >> parameter is used to pass the pointer of the previously embedded >> structure of the shrinker instance. >> >> 2. shrinker_free() >> >> Used to free the shrinker instance when the registration of shrinker >> fails. >> >> 3. unregister_and_free_shrinker() >> >> Used to unregister and free the shrinker instance, and the kfree() >> will be changed to kfree_rcu() later. >> >> Signed-off-by: Qi Zheng >> --- >> include/linux/shrinker.h | 12 ++++++++++++ >> mm/vmscan.c | 35 +++++++++++++++++++++++++++++++++++ >> 2 files changed, 47 insertions(+) >> >> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h >> index 43e6fcabbf51..8e9ba6fa3fcc 100644 >> --- a/include/linux/shrinker.h >> +++ b/include/linux/shrinker.h >> @@ -107,6 +107,18 @@ extern void unregister_shrinker(struct shrinker *shrinker); >> extern void free_prealloced_shrinker(struct shrinker *shrinker); >> extern void synchronize_shrinkers(void); >> >> +typedef unsigned long (*count_objects_cb)(struct shrinker *s, >> + struct shrink_control *sc); >> +typedef unsigned long (*scan_objects_cb)(struct shrinker *s, >> + struct shrink_control *sc); >> + >> +struct shrinker *shrinker_alloc_and_init(count_objects_cb count, >> + scan_objects_cb scan, long batch, >> + int seeks, unsigned flags, >> + void *priv_data); >> +void shrinker_free(struct shrinker *shrinker); >> +void unregister_and_free_shrinker(struct shrinker *shrinker); > > Hmmmm. Not exactly how I envisioned this to be done. > > Ok, this will definitely work, but I don't think it is an > improvement. It's certainly not what I was thinking of when I > suggested dynamically allocating shrinkers. > > The main issue is that this doesn't simplify the API - it expands it > and creates a minefield of old and new functions that have to be > used in exactly the right order for the right things to happen. > > What I was thinking of was moving the entire shrinker setup code > over to the prealloc/register_prepared() algorithm, where the setup > is already separated from the activation of the shrinker. > > That is, we start by renaming prealloc_shrinker() to > shrinker_alloc(), adding a flags field to tell it everything that it > needs to alloc (i.e. the NUMA/MEMCG_AWARE flags) and having it > returned a fully allocated shrinker ready to register. Initially > this also contains an internal flag to say the shrinker was > allocated so that unregister_shrinker() knows to free it. > > The caller then fills out the shrinker functions, seeks, etc. just > like the do now, and then calls register_shrinker_prepared() to make > the shrinker active when it wants to turn it on. > > When it is time to tear down the shrinker, no API needs to change. > unregister_shrinker() does all the shutdown and frees all the > internal memory like it does now. If the shrinker is also marked as > allocated, it frees the shrinker via RCU, too. > > Once everything is converted to this API, we then remove > register_shrinker(), rename register_shrinker_prepared() to > shrinker_register(), rename unregister_shrinker to > shrinker_unregister(), get rid of the internal "allocated" flag > and always free the shrinker. IIUC, you mean that we also need to convert the original statically defined shrinker instances to dynamically allocated. I think this is a good idea, it helps to simplify the APIs and also remove special handling for case a and b (mentioned in cover letter). > > At the end of the patchset, every shrinker should be set > up in a manner like this: > > > sb->shrinker = shrinker_alloc(SHRINKER_MEMCG_AWARE|SHRINKER_NUMA_AWARE, > "sb-%s", type->name); > if (!sb->shrinker) > return -ENOMEM; > > sb->shrinker->count_objects = super_cache_count; > sb->shrinker->scan_objects = super_cache_scan; > sb->shrinker->batch = 1024; > sb->shrinker->private = sb; > > ..... > > shrinker_register(sb->shrinker); > > And teardown is just a call to shrinker_unregister(sb->shrinker) > as it is now. > > i.e. the entire shrinker regsitration API is now just three > functions, down from the current four, and much simpler than the > the seven functions this patch set results in... > > The other advantage of this is that it will break all the existing > out of tree code and third party modules using the old API and will > no longer work with a kernel using lockless slab shrinkers. They > need to break (both at the source and binary levels) to stop bad > things from happening due to using uncoverted shrinkers in the new > setup. Got it. And totally agree. I will do it in the v2. Thanks, Qi > > -Dave.