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

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Date:   Fri, 7 Jan 2022 15:44:50 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Hillf Danton <hdanton@...a.com>, Jens Axboe <axboe@...nel.dk>,
        Jesse Barnes <jsbarnes@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Michael Larabel <Michael@...haellarabel.com>,
        Rik van Riel <riel@...riel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Will Deacon <will@...nel.org>,
        Ying Huang <ying.huang@...el.com>,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        page-reclaim@...gle.com, x86@...nel.org,
        Konstantin Kharlamov <Hi-Angel@...dex.ru>
Subject: Re: [PATCH v6 6/9] mm: multigenerational lru: aging

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
> +		if (!mmap_read_trylock(mm))
> +			goto contended;

Have you evaluated the behavior under mmap_sem contention? I mean what
would be an effect of some mms being excluded from the walk? This path
is called from direct reclaim and we do allocate with exclusive mmap_sem
IIRC and the trylock can fail in a presence of pending writer if I am
not mistaken so even the read lock holder (e.g. an allocation from the #PF)
can bypass the walk.

Or is this considered statistically insignificant thus a theoretical
problem?
-- 
Michal Hocko
SUSE Labs