On 2021/7/3 11:35, Theodore Ts'o wrote:
> On Sat, Jul 03, 2021 at 11:05:07AM +0800, Zhang Yi wrote:
>>
>> Originally, I want to add this shrinker as a optional feature for jbd2 because
>> only ext4 use it now and I'm not sure does ocfs2 needs this feature. So I export
>> jbd2_journal_[un]register_shrinker(), ext4 could invoke them individually.
>
> The reason why bdev_try_to_free_page() callback was needed for ext4
> --- namely so there was a way to release checkpointed buffers under
> memory pressure --- also exists for ocfs2. It was probably true that
> in most deployments of ocfs2, they weren't running with super-tight
> memory availability, so it may not have been necessary the same way
> that it might be necessary, say, if ext4 was being used on a Rasberry
> Pi. :-)
>
>> And one more thing we to could do is rename the 'j_jh_shrink_count' to something
>> like 'j_checkpoint_jh_count' because we always init it no matter we register the
>> shrinker or not later.
>
> That makes sense.
>
> In fact, unless I'm mistaken, I don't think it's legal to call
> percpu_counter_{inc,dec} if the shrinker isn't initialized. So for
> ocfs2, if we didn't initialize percpu_counter, when
> __jbd2_journal_insert_checkpoint() tries to call percpu_counter_inc(),
> I believe things would potentially go *boom* on some implementations
> of the percpu counter (e.g., on Power and ARM). So not only would it
> not hurt to register the shrinker for ocfs2, I think it's required.
>
> So yeah, let's rename it to something like j_checkpoint_jh_count, and
> then let's inline jbd2_journal_[un]register_shrinker() in
> journal_init_common() and jbd2_journal_unregister_shrinker().
>
> What do you think?
>
Yeah, it sounds good to me. Do you want me to send the fix patch, or you
modify your commit 8f9e16badb8fd in another email directly?
Thanks,
Yi.
