linux-kernel - Re: [PATCH] memory-hotplug: don't BUG() in register_memory

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Message-Id: <20151221150649.f385889426082059bfc09495@linux-foundation.org>
Date:	Mon, 21 Dec 2015 15:06:49 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Tang Chen <tangchen@...fujitsu.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Xishi Qiu <qiuxishi@...wei.com>,
	Sheng Yong <shengyong1@...wei.com>,
	David Rientjes <rientjes@...gle.com>,
	Zhu Guihua <zhugh.fnst@...fujitsu.com>,
	Dan Williams <dan.j.williams@...el.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Igor Mammedov <imammedo@...hat.com>
Subject: Re: [PATCH] memory-hotplug: don't BUG() in
 register_memory_resource()

On Mon, 21 Dec 2015 11:13:15 +0100 Vitaly Kuznetsov <vkuznets@...hat.com> wrote:

> Andrew Morton <akpm@...ux-foundation.org> writes:
> 
> > On Fri, 18 Dec 2015 15:50:24 +0100 Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
> >
> >> Out of memory condition is not a bug and while we can't add new memory in
> >> such case crashing the system seems wrong. Propagating the return value
> >> from register_memory_resource() requires interface change.
> >> 
> >> --- a/mm/memory_hotplug.c
> >> +++ b/mm/memory_hotplug.c
> >> +static int register_memory_resource(u64 start, u64 size,
> >> +				    struct resource **resource)
> >>  {
> >>  	struct resource *res;
> >>  	res = kzalloc(sizeof(struct resource), GFP_KERNEL);
> >> -	BUG_ON(!res);
> >> +	if (!res)
> >> +		return -ENOMEM;
> >>  
> >>  	res->name = "System RAM";
> >>  	res->start = start;
> >> @@ -140,9 +142,10 @@ static struct resource *register_memory_resource(u64 start, u64 size)
> >>  	if (request_resource(&iomem_resource, res) < 0) {
> >>  		pr_debug("System RAM resource %pR cannot be added\n", res);
> >>  		kfree(res);
> >> -		res = NULL;
> >> +		return -EEXIST;
> >>  	}
> >> -	return res;
> >> +	*resource = res;
> >> +	return 0;
> >>  }
> >
> > Was there a reason for overwriting the request_resource() return
> > value?
> > Ordinarily it should be propagated back to callers.
> >
> > Please review.
> >
> 
> This is a nice-to-have addition but it will break at least ACPI
> memhotplug: request_resource() has the following:
> 
> conflict = request_resource_conflict(root, new);
> return conflict ? -EBUSY : 0;
> 
> so we'll end up returning -EBUSY from register_memory_resource() and
> add_memory(), at the same time acpi_memory_enable_device() counts on
> -EEXIST:
> 
> result = add_memory(node, info->start_addr, info->length);
> 
> /*
> * If the memory block has been used by the kernel, add_memory()
> * returns -EEXIST. If add_memory() returns the other error, it
> * means that this memory block is not used by the kernel.
> */
> if (result && result != -EEXIST)
> continue;
> 
> So I see 3 options here:
> 1) Keep the overwrite
> 2) Change the request_resource() return value to -EEXIST
> 3) Adapt all add_memory() call sites to -EBUSY.
> 
> Please let me know your preference.

urgh, what a mess.  We should standardize on EBUSY or EEXIST, I don't
see that it matter much which is chosen.  And for robustness the
callers should be checking for (err < 0) unless there's a very good
reason otherwise.

But it doesn't seem terribly important.
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