Date:   Wed, 12 Jan 2022 11:17:53 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Yu Zhao <yuzhao@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andi Kleen <ak@linux.intel.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Hillf Danton <hdanton@sina.com>, Jens Axboe <axboe@kernel.dk>,
        Jesse Barnes <jsbarnes@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Matthew Wilcox <willy@infradead.org>,
        Mel Gorman <mgorman@suse.de>,
        Michael Larabel <Michael@michaellarabel.com>,
        Rik van Riel <riel@surriel.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Will Deacon <will@kernel.org>,
        Ying Huang <ying.huang@intel.com>,
        linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        page-reclaim@google.com, x86@kernel.org,
        Konstantin Kharlamov <Hi-Angel@yandex.ru>
Subject: Re: [PATCH v6 6/9] mm: multigenerational lru: aging
Message-ID: <Yd6q0QdLVTS53vu4@dhcp22.suse.cz>
References: <20220104202227.2903605-1-yuzhao@google.com>
 <20220104202227.2903605-7-yuzhao@google.com>
 <Ydxlg5rI4ZvODQvF@dhcp22.suse.cz>
 <Yd4oaUwHkpadAKwe@google.com>
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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. Is there any reason you are not using folio_memcg_lock in the
pte walk instead?
-- 
Michal Hocko
SUSE Labs