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From:   Andi Kleen <ak@linux.intel.com>
To:     Yu Zhao <yuzhao@google.com>
Cc:     linux-mm@kvack.org, Alex Shi <alex.shi@linux.alibaba.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Hillf Danton <hdanton@sina.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Matthew Wilcox <willy@infradead.org>,
        Mel Gorman <mgorman@suse.de>, Michal Hocko <mhocko@suse.com>,
        Roman Gushchin <guro@fb.com>, Vlastimil Babka <vbabka@suse.cz>,
        Wei Yang <richard.weiyang@linux.alibaba.com>,
        Yang Shi <shy828301@gmail.com>,
        Ying Huang <ying.huang@intel.com>,
        linux-kernel@vger.kernel.org, page-reclaim@google.com
Subject: Re: [PATCH v1 10/14] mm: multigenerational lru: core
References: <20210313075747.3781593-1-yuzhao@google.com>
        <20210313075747.3781593-11-yuzhao@google.com>
Date:   Sun, 14 Mar 2021 19:02:01 -0700
In-Reply-To: <20210313075747.3781593-11-yuzhao@google.com> (Yu Zhao's message
        of "Sat, 13 Mar 2021 00:57:43 -0700")
Message-ID: <871rchw4x2.fsf@linux.intel.com>
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Yu Zhao <yuzhao@google.com> writes:
> +
> +#ifdef CONFIG_MEMCG
> +		if (memcg && atomic_read(&memcg->moving_account))
> +			goto contended;
> +#endif
> +		if (!mmap_read_trylock(mm))
> +			goto contended;

These are essentially spinloops. Surely you need a cpu_relax() somewhere?

In general for all of spinloop like constructs it would be useful to
consider how to teach lockdep about them.

> +	do {
> +		old_flags = READ_ONCE(page->flags);
> +		new_gen = ((old_flags & LRU_GEN_MASK) >> LRU_GEN_PGOFF) - 1;
> +		VM_BUG_ON_PAGE(new_gen < 0, page);
> +		if (new_gen >= 0 && new_gen != old_gen)
> +			goto sort;
> +
> +		new_gen = (old_gen + 1) % MAX_NR_GENS;
> +		new_flags = (old_flags & ~LRU_GEN_MASK) | ((new_gen + 1UL) << LRU_GEN_PGOFF);
> +		/* mark page for reclaim if pending writeback */
> +		if (front)
> +			new_flags |= BIT(PG_reclaim);
> +	} while (cmpxchg(&page->flags, old_flags, new_flags) !=
> old_flags);

I see this cmpxchg flags pattern a lot. Could there be some common code
factoring?

-Andi