Re: [PATCH] perf/core: fix mlock accounting in perf_mmap()

[Song Liu]

> On Jan 23, 2020, at 1:19 AM, Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx> wrote:
> 
> Song Liu <songliubraving@xxxxxx> writes:
> 
>>> On Jan 20, 2020, at 12:24 AM, Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx> wrote:
>>> 
>>> Song Liu <songliubraving@xxxxxx> writes:
>>> 
>>>> sysctl_perf_event_mlock and user->locked_vm can change value
>>>> independently, so we can't guarantee:
>>>> 
>>>>   user->locked_vm <= user_lock_limit
>>> 
>>> This means: if the sysctl got sufficiently decreased, so that the
>>> existing locked_vm exceeds it, we need to deal with the overflow, right?
>> 
>> Reducing sysctl is one way to generate the overflow. Another way is to 
>> call setrlimit() from user space to allow bigger user->locked_vm. 
> 
> You mean RLIMIT_MEMLOCK? That's a limit on mm->pinned_vm. Doesn't affect
> user->locked_vm.

This depends. For example, bpf_charge_memlock() uses RLIMIT_MEMLOCK as the
limit for user->locked_vm. This makes sense, because the bpf map created by 
a process may stay longer than the process. 

Thanks,
Song