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Date: Thu, 1 Sep 2016 13:37:17 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Reza Arbab <arbab@linux.vnet.ibm.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        David Rientjes <rientjes@google.com>,
        Yaowei Bai <baiyaowei@cmss.chinamobile.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Xishi Qiu <qiuxishi@huawei.com>,
        David Vrabel <david.vrabel@citrix.com>, Chen Yucong <slaoub@gmail.com>,
        Andrew Banman <abanman@sgi.com>, Seth Jennings <sjenning@redhat.com>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] memory-hotplug: fix store_mem_state() return value
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On Thu,  1 Sep 2016 10:29:37 -0500 Reza Arbab <arbab@linux.vnet.ibm.com> wrote:

> If store_mem_state() is called to online memory which is already online,
> it will return 1, the value it got from device_online().
> 
> This is wrong because store_mem_state() is a device_attribute .store
> function. Thus a non-negative return value represents input bytes read.
> 
> Set the return value to -EINVAL in this case.
> 

I actually made the mistake of reading this code.

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.

And now I don't understand this patch.  Because:

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?

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?  Well, that depends on the bus_type.online rules which
appear to be undocumented.  What is the bus implementation supposed to
do when a request is made to online an already-online device?