Re: [PATCH v6 6/9] mm: multigenerational lru: aging

[Yu Zhao]

On Wed, Jan 12, 2022 at 11:17:53AM +0100, Michal Hocko wrote:
> On Tue 11-01-22 18:01:29, Yu Zhao wrote:
> > On Mon, Jan 10, 2022 at 05:57:39PM +0100, Michal Hocko wrote:
> > > On Tue 04-01-22 13:22:25, Yu Zhao wrote:
> > > [...]
> > > > +static void walk_mm(struct lruvec *lruvec, struct mm_struct *mm, struct lru_gen_mm_walk *walk)
> > > > +{
> > > > +	static const struct mm_walk_ops mm_walk_ops = {
> > > > +		.test_walk = should_skip_vma,
> > > > +		.p4d_entry = walk_pud_range,
> > > > +	};
> > > > +
> > > > +	int err;
> > > > +#ifdef CONFIG_MEMCG
> > > > +	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
> > > > +#endif
> > > > +
> > > > +	walk->next_addr = FIRST_USER_ADDRESS;
> > > > +
> > > > +	do {
> > > > +		unsigned long start = walk->next_addr;
> > > > +		unsigned long end = mm->highest_vm_end;
> > > > +
> > > > +		err = -EBUSY;
> > > > +
> > > > +		rcu_read_lock();
> > > > +#ifdef CONFIG_MEMCG
> > > > +		if (memcg && atomic_read(&memcg->moving_account))
> > > > +			goto contended;
> > > > +#endif
> > > 
> > > Why do you need to check for moving_account?
> > 
> > This check, if succeeds, blocks memcg migration.
> 
> OK, I can see that you rely on the RCU here for the synchronization. A
> comment which mentions mem_cgroup_move_charge would be helpful for
> clarity.

Will do

> Is there any reason you are not using folio_memcg_lock in the
> pte walk instead?

We have a particular lruvec (the first arg), hence a particular memcg
to lock. But we don't have a particular page to lock.