Re: [PATCH v2] mm: do not try to migrate lru_gen if it's not associated with a memcg

[msizanoen <msizanoen@qtmlabs.xyz>]

On 1/16/23 06:13, Yu Zhao wrote:
> On Sun, Jan 15, 2023 at 6:47 AM msizanoen1 <msizanoen@qtmlabs.xyz> wrote:
>> In some cases, memory cgroup migration can be initiated by userspace
>> right after a process was created and right before `lru_gen_add_mm()` is
>> called (e.g. by some program watching a cgroup and moving away any
>> processes it detects[1]), which results in the following sequence of
>> WARNs followed by an Oops as the kernel attempts to perform a
>> `lru_gen_add_mm()` twice on the same `mm`:
> ...
>
>> Fix this by simply leaving the lru_gen alone if it has not been
>> associated with a memcg yet, as it should eventually be assigned to the
>> right cgroup anyway.
>>
>> [1]: https://gitlab.freedesktop.org/benzea/uresourced/-/blob/master/cgroupify/cgroupify.c
>>
>> v2:
>>          Added stable cc tags
>>
>> Signed-off-by: N/A (patch should not be copyrightable)
>> Cc: stable@vger.kernel.org
> Thanks for the fix.  Cc'ing stable is the right thing to do. The
> commit message and the comment styles could be easily adjusted to
> align with the guidelines.
>
> I don't think the N/A is acceptible though. I fully respect it if you
> wish to remain anonymous -- I can send a similar fix crediting you
> as the "anonymous user <msizanoen@qtmlabs.xyz>" who reported this bug.
Sure, just add my email in the `Reported-by: ` and `Tested-by: ` lines 
and git-send-email should automatically add me to the Cc list.
>
> A bit of background on how I broke it: an old version I have on 4.15
> calls lru_gen_add_mm() before cgroup_post_fork(), which excludes
> cgroup migrations by cgroup_threadgroup_rwsem. When I rebased it, I
> made lru_gen_add_mm() depend on task_lock for the synchronization with
> cgroup migrations -- the decoupling seemed (still seems) to make it
> less complicated -- but this is not safe unless we have the check below.
>
>
>
>
>> ---
>>   mm/vmscan.c | 8 +++++++-
>>   1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index bd6637fcd8f9..0cac40e7484c 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -3323,13 +3323,19 @@ void lru_gen_migrate_mm(struct mm_struct *mm)
>>          if (mem_cgroup_disabled())
>>                  return;
>>
>> +       /* This could happen if cgroup migration is invoked before the process
>> +        * lru_gen is associated with a memcg (e.g. during process creation).
>> +        * Simply ignore it in this case as the lru_gen will get assigned the
>> +        * right cgroup later. */
>> +       if (!mm->lru_gen.memcg)
>> +               return;
>> +
>>          rcu_read_lock();
>>          memcg = mem_cgroup_from_task(task);
>>          rcu_read_unlock();
>>          if (memcg == mm->lru_gen.memcg)
>>                  return;
>>
>> -       VM_WARN_ON_ONCE(!mm->lru_gen.memcg);
>>          VM_WARN_ON_ONCE(list_empty(&mm->lru_gen.list));
>>
>>          lru_gen_del_mm(mm);