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Date: Wed, 31 Aug 2016 14:06:14 -0700 (PDT)
From: David Rientjes <rientjes@google.com>
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To: Andrew Morton <akpm@linux-foundation.org>
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        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
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        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>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [RESEND PATCH v2] memory-hotplug: fix store_mem_state() return
 value
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On Wed, 31 Aug 2016, Andrew Morton wrote:

> > Attempting to online memory which is already online will cause this:
> > 
> > 1. store_mem_state() called with buf="online"
> > 2. device_online() returns 1 because device is already online
> > 3. store_mem_state() returns 1
> > 4. calling code interprets this as 1-byte buffer read
> > 5. store_mem_state() called again with buf="nline"
> > 6. store_mem_state() returns -EINVAL
> > 
> > Example:
> > 
> > $ cat /sys/devices/system/memory/memory0/state
> > online
> > $ echo online > /sys/devices/system/memory/memory0/state
> > -bash: echo: write error: Invalid argument
> > 
> > Fix the return value of store_mem_state() so this doesn't happen.
> 
> So..  what *does* happen after the patch?  Is some sort of failure still
> reported?  Or am I correct in believing that the operation will appear
> to have succeeded?  If so, is that desirable?
> 

It's not desirable, before commit 4f3549d72 this would have returned 
EINVAL since __memory_block_change_state() does not see the state as 
MEM_OFFLINE when the write is done.  The correct fix is for 
store_mem_state() to return -EINVAL when device_online() returns non-zero.