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

Qi Zheng
2023-06-24T11:09:02Z

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