On Sat, Jul 03, 2021 at 11:05:07AM +0800, Zhang Yi wrote:

quoted

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.  :-)

quoted

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?