linux-ext4 - Re: [PATCH 24/29] mm: vmscan: make global slab shrink lockless

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Date:   Sat, 24 Jun 2023 19:08:18 +0800
From:   Qi Zheng <zhengqi.arch@...edance.com>
To:     Dave Chinner <david@...morbit.com>, paulmck@...nel.org
Cc:     Vlastimil Babka <vbabka@...e.cz>, akpm@...ux-foundation.org,
        tkhai@...ru, roman.gushchin@...ux.dev, djwong@...nel.org,
        brauner@...nel.org, tytso@....edu, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, intel-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org,
        dm-devel@...hat.com, linux-raid@...r.kernel.org,
        linux-bcache@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-nfs@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-btrfs@...r.kernel.org
Subject: Re: [PATCH 24/29] mm: vmscan: make global slab shrink lockless

Hi Dave,

On 2023/6/24 06:19, Dave Chinner wrote:
> On Fri, Jun 23, 2023 at 09:10:57PM +0800, Qi Zheng wrote:
>> On 2023/6/23 14:29, Dave Chinner wrote:
>>> On Thu, Jun 22, 2023 at 05:12:02PM +0200, Vlastimil Babka wrote:
>>>> On 6/22/23 10:53, Qi Zheng wrote:
>>> Yes, I suggested the IDR route because radix tree lookups under RCU
>>> with reference counted objects are a known safe pattern that we can
>>> easily confirm is correct or not.  Hence I suggested the unification
>>> + IDR route because it makes the life of reviewers so, so much
>>> easier...
>>
>> In fact, I originally planned to try the unification + IDR method you
>> suggested at the beginning. But in the case of CONFIG_MEMCG disabled,
>> the struct mem_cgroup is not even defined, and root_mem_cgroup and
>> shrinker_info will not be allocated.  This required more code changes, so
>> I ended up keeping the shrinker_list and implementing the above pattern.
> 
> Yes. Go back and read what I originally said needed to be done
> first. In the case of CONFIG_MEMCG=n, a dummy root memcg still needs
> to exist that holds all of the global shrinkers. Then shrink_slab()
> is only ever passed a memcg that should be iterated.
> 
> Yes, it needs changes external to the shrinker code itself to be
> made to work. And even if memcg's are not enabled, we can still use
> the memcg structures to ensure a common abstraction is used for the
> shrinker tracking infrastructure....

Yeah, what I imagined before was to define a more concise struct
mem_cgroup in the case of CONFIG_MEMCG=n, then allocate a dummy root
memcg on system boot:

#ifdef !CONFIG_MEMCG

struct shrinker_info {
	struct rcu_head rcu;
	atomic_long_t *nr_deferred;
	unsigned long *map;
	int map_nr_max;
};

struct mem_cgroup_per_node {
	struct shrinker_info __rcu	*shrinker_info;
};

struct mem_cgroup {
	struct mem_cgroup_per_node *nodeinfo[];
};

#endif

But I have a concern: if all global shrinkers are tracking with the
info->map of root memcg, a shrinker->id needs to be assigned to them,
which will cause info->map_nr_max to become larger than before, then
making the traversal of info->map slower.

> 
>> If the above pattern is not safe, I will go back to the unification +
>> IDR method.
> 
> And that is exactly how we got into this mess in the first place....

I only found one similar pattern in the kernel:

fs/smb/server/oplock.c:find_same_lease_key/smb_break_all_levII_oplock/lookup_lease_in_table

But IIUC, the refcount here needs to be decremented after holding
rcu lock as I did above.

So regardless of whether we choose unification + IDR in the end, I still
want to confirm whether the pattern I implemented above is safe. :)

Thanks,
Qi

> 
> -Dave