linux-kernel - Re: [PATCH v3] memory-hotplug: fix store_mem

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Date:   Thu, 1 Sep 2016 16:45:53 -0500
From:   Reza Arbab <arbab@...ux.vnet.ibm.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Yaowei Bai <baiyaowei@...s.chinamobile.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Dan Williams <dan.j.williams@...el.com>,
        Xishi Qiu <qiuxishi@...wei.com>,
        David Vrabel <david.vrabel@...rix.com>,
        Chen Yucong <slaoub@...il.com>,
        Andrew Banman <abanman@....com>,
        Seth Jennings <sjenning@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] memory-hotplug: fix store_mem_state() return value

On Thu, Sep 01, 2016 at 01:37:17PM -0700, Andrew Morton wrote:
>What the heck are the return value semantics of bus_type.online?
>Sometimes 0, sometimes 1 and apparently sometimes -Efoo values.  What
>are these things trying to tell the caller and why is "1" ever useful
>and why doesn't anyone document anything.  grr.

You might be getting tangled in the two codepaths the way I was.

If you do 'echo 1 > online':
	dev_attr_store
		online_store
			device_online
				memory_subsys_online
					memory_block_change_state

If you do 'echo online > state':
	dev_attr_store
		store_mem_state
			device_online
				memory_subsys_online
					memory_block_change_state

>static int memory_subsys_online(struct device *dev)
>{
>	struct memory_block *mem = to_memory_block(dev);
>	int ret;
>
>	if (mem->state == MEM_ONLINE)
>		return 0;
>
>Doesn't that "return 0" contradict the changelog?

The online-to-online check being used is higher in the call chain:

int device_online(struct device *dev)
{
	if (device_supports_offline(dev)) {
		if (dev->offline) {
			...
		} else {
			ret = 1;
		}
	}

>Also, is store_mem_state() the correct place to fix this?  Instead,
>should memory_block_change_state() detect an attempt to online
>already-online memory and itself return -EINVAL, and permit that to be
>propagated back?

Doing that would affect both codepaths, and as David made clear, would 
break backwards compatibility because their established behaviors are 
different.

'echo 1 > online' returns 0 if the device is already online
'echo online > state' returns -EINVAL if the device is already online

-- 
Reza Arbab