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Subject: Re: [v5 2/3] mm/hotplug: make remove_memory() interface useable
To:     Pavel Tatashin <pasha.tatashin@soleen.com>, jmorris@namei.org,
        sashal@kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-nvdimm@lists.01.org,
        akpm@linux-foundation.org, mhocko@suse.com,
        dave.hansen@linux.intel.com, dan.j.williams@intel.com,
        keith.busch@intel.com, vishal.l.verma@intel.com,
        dave.jiang@intel.com, zwisler@kernel.org, thomas.lendacky@amd.com,
        ying.huang@intel.com, fengguang.wu@intel.com, bp@suse.de,
        bhelgaas@google.com, baiyaowei@cmss.chinamobile.com, tiwai@suse.de,
        jglisse@redhat.com, david@redhat.com
References: <20190502184337.20538-1-pasha.tatashin@soleen.com>
 <20190502184337.20538-3-pasha.tatashin@soleen.com>
From:   Dave Hansen <dave.hansen@intel.com>
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Message-ID: <cac721ed-c404-19d1-71d1-37c66df9b2a8@intel.com>
Date:   Mon, 6 May 2019 10:57:28 -0700
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> -static inline void remove_memory(int nid, u64 start, u64 size) {}
> +static inline bool remove_memory(int nid, u64 start, u64 size)
> +{
> +	return -EBUSY;
> +}

This seems like an appropriate place for a WARN_ONCE(), if someone
manages to call remove_memory() with hotplug disabled.

BTW, I looked and can't think of a better errno, but -EBUSY probably
isn't the best error code, right?

> -void remove_memory(int nid, u64 start, u64 size)
> +/**
> + * remove_memory
> + * @nid: the node ID
> + * @start: physical address of the region to remove
> + * @size: size of the region to remove
> + *
> + * NOTE: The caller must call lock_device_hotplug() to serialize hotplug
> + * and online/offline operations before this call, as required by
> + * try_offline_node().
> + */
> +void __remove_memory(int nid, u64 start, u64 size)
>  {
> +
> +	/*
> +	 * trigger BUG() is some memory is not offlined prior to calling this
> +	 * function
> +	 */
> +	if (try_remove_memory(nid, start, size))
> +		BUG();
> +}

Could we call this remove_offline_memory()?  That way, it makes _some_
sense why we would BUG() if the memory isn't offline.