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SubjectRe: [v6 PATCH 09/11] mm: vmscan: don't need allocate shrinker->nr_deferred for memcg aware shrinkers
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On 04.02.2021 20:32, Yang Shi wrote:
> On Thu, Feb 4, 2021 at 2:14 AM Kirill Tkhai <ktkhai@virtuozzo.com> wrote:
>>
>> On 04.02.2021 12:29, Kirill Tkhai wrote:
>>> On 03.02.2021 20:20, Yang Shi wrote:
>>>> Now nr_deferred is available on per memcg level for memcg aware shrinkers, so don't need
>>>> allocate shrinker->nr_deferred for such shrinkers anymore.
>>>>
>>>> The prealloc_memcg_shrinker() would return -ENOSYS if !CONFIG_MEMCG or memcg is disabled
>>>> by kernel command line, then shrinker's SHRINKER_MEMCG_AWARE flag would be cleared.
>>>> This makes the implementation of this patch simpler.
>>>>
>>>> Acked-by: Vlastimil Babka <vbabka@suse.cz>
>>>> Signed-off-by: Yang Shi <shy828301@gmail.com>
>>>> ---
>>>> mm/vmscan.c | 31 ++++++++++++++++---------------
>>>> 1 file changed, 16 insertions(+), 15 deletions(-)
>>>>
>>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>>> index 545422d2aeec..20a35d26ae12 100644
>>>> --- a/mm/vmscan.c
>>>> +++ b/mm/vmscan.c
>>>> @@ -334,6 +334,9 @@ static int prealloc_memcg_shrinker(struct shrinker *shrinker)
>>>> {
>>>> int id, ret = -ENOMEM;
>>>>
>>>> + if (mem_cgroup_disabled())
>>>> + return -ENOSYS;
>>>> +
>>>> down_write(&shrinker_rwsem);
>>>> /* This may call shrinker, so it must use down_read_trylock() */
>>>> id = idr_alloc(&shrinker_idr, shrinker, 0, 0, GFP_KERNEL);
>>>> @@ -414,7 +417,7 @@ static bool writeback_throttling_sane(struct scan_control *sc)
>>>> #else
>>>> static int prealloc_memcg_shrinker(struct shrinker *shrinker)
>>>> {
>>>> - return 0;
>>>> + return -ENOSYS;
>>>> }
>>>>
>>>> static void unregister_memcg_shrinker(struct shrinker *shrinker)
>>>> @@ -525,8 +528,18 @@ unsigned long lruvec_lru_size(struct lruvec *lruvec, enum lru_list lru, int zone
>>>> */
>>>> int prealloc_shrinker(struct shrinker *shrinker)
>>>> {
>>>> - unsigned int size = sizeof(*shrinker->nr_deferred);
>>>> + unsigned int size;
>>>> + int err;
>>>> +
>>>> + if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
>>>> + err = prealloc_memcg_shrinker(shrinker);
>>>> + if (err != -ENOSYS)
>>>> + return err;
>>>>
>>>> + shrinker->flags &= ~SHRINKER_MEMCG_AWARE;
>>>> + }
>>>> +
>>>> + size = sizeof(*shrinker->nr_deferred);
>>>> if (shrinker->flags & SHRINKER_NUMA_AWARE)
>>>> size *= nr_node_ids;
>>>
>>> This may sound surprisingly, but IIRC do_shrink_slab() may be called on early boot
>>> *even before* root_mem_cgroup is allocated. AFAIR, I received syzcaller crash report
>>> because of this, when I was implementing shrinker_maps.
>>>
>>> This is a reason why we don't use shrinker_maps even in case of mem cgroup is not
>>> disabled: we iterate every shrinker of shrinker_list. See check in shrink_slab():
>>>
>>> if (!mem_cgroup_disabled() && !mem_cgroup_is_root(memcg))
>>>
>>> Possible, we should do the same for nr_deferred: 1)always allocate shrinker->nr_deferred,
>>> 2)use shrinker->nr_deferred in count_nr_deferred() and set_nr_deferred().
>>
>> I looked over my mail box, and I can't find that crash report and conditions to reproduce.
>>
>> Hm, let's remain this as is, and we rework this in case of such early shrinker call is still
>> possible, and there will be a report...
>
> Sure. But I'm wondering how that could happen. On a very small machine?

Sorry, but I don't remember. Maybe this case you said. Maybe some self-tests on node boot..

>>
>> Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>
>>
>> With only nit:
>>
>>>>
>>>> @@ -534,26 +547,14 @@ int prealloc_shrinker(struct shrinker *shrinker)
>>>> if (!shrinker->nr_deferred)
>>>> return -ENOMEM;
>>>>
>>>> - if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
>>>> - if (prealloc_memcg_shrinker(shrinker))
>>>> - goto free_deferred;
>>>> - }
>>>>
>>>> return 0;
>>>> -
>>>> -free_deferred:
>>>> - kfree(shrinker->nr_deferred);
>>>> - shrinker->nr_deferred = NULL;
>>>> - return -ENOMEM;
>>>> }
>>>>
>>>> void free_prealloced_shrinker(struct shrinker *shrinker)
>>>> {
>>>> - if (!shrinker->nr_deferred)
>>>> - return;
>>>> -
>>>> if (shrinker->flags & SHRINKER_MEMCG_AWARE)
>>>> - unregister_memcg_shrinker(shrinker);
>>>> + return unregister_memcg_shrinker(shrinker);
>>
>> I've never seen return of void function in linux kernel. I'm not sure this won't confuse people.
>
> Will fix in v7.
>
>>
>>>>
>>>> kfree(shrinker->nr_deferred);
>>>> shrinker->nr_deferred = NULL;
>>>>
>>>
>>
>>

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