linux-kernel - Re: [PATCH -V3 4/4] dax, kmem: calculate abstract distance with general interface

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Date:   Tue, 19 Sep 2023 15:31:29 +1000
From:   Alistair Popple <apopple@...dia.com>
To:     Huang Ying <ying.huang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Bharata B Rao <bharata@....com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Wei Xu <weixugc@...gle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Michal Hocko <mhocko@...nel.org>,
        Yang Shi <shy828301@...il.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Rafael J Wysocki <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH -V3 4/4] dax, kmem: calculate abstract distance with
 general interface


Huang Ying <ying.huang@...el.com> writes:

> diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
> index fa1a8b418f9a..ca68ef17554b 100644
> --- a/mm/memory-tiers.c
> +++ b/mm/memory-tiers.c
> @@ -586,7 +586,7 @@ EXPORT_SYMBOL_GPL(init_node_memory_type);
>  void clear_node_memory_type(int node, struct memory_dev_type *memtype)
>  {
>  	mutex_lock(&memory_tier_lock);
> -	if (node_memory_types[node].memtype == memtype)
> +	if (node_memory_types[node].memtype == memtype || !memtype)
>  		node_memory_types[node].map_count--;
>
>  	/*
>  	 * If we umapped all the attached devices to this node,

This implies it's possible memtype == NULL. Yet we have this:

	 * clear the node memory type.
	 */
	if (!node_memory_types[node].map_count) {
		node_memory_types[node].memtype = NULL;
		put_memory_type(memtype);
	}

It's not safe to call put_memory_type(NULL), so what condition guarantees
map_count > 1 when called with memtype == NULL? Thanks.

 - Alistair