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

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Message-ID: <57C8D71F.1080803@huawei.com>
Date:   Fri, 2 Sep 2016 09:34:23 +0800
From:   Xishi Qiu <qiuxishi@...wei.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     Reza Arbab <arbab@...ux.vnet.ibm.com>,
        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>,
        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 2016/9/2 4:37, Andrew Morton wrote:

> On Thu,  1 Sep 2016 10:29:37 -0500 Reza Arbab <arbab@...ux.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;
> 

I think we will not execute here, it will return from device_online(),
because "if (dev->offline)" is false and return 1.

But the two return vaules are different if we do online-to-online.
memory_subsys_online() return 0, and device_online() return 1,
this is a little confusion.

When device_online() return 1, online_store() return 1 and store_mem_state()
return -EINVAL even without this patch, as Reza described in v2.

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

Thanks,
Xishi Qiu

> 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?
> 
> 
> 
> .
>