linux-kernel - Re: [PATCH 2/4] mm/memory-hotplug: allow memory resources to be children

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Date:   Fri, 18 Jan 2019 11:58:54 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jerome Glisse <jglisse@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     dave@...1.net, dan.j.williams@...el.com, dave.jiang@...el.com,
        zwisler@...nel.org, vishal.l.verma@...el.com,
        thomas.lendacky@....com, akpm@...ux-foundation.org,
        mhocko@...e.com, linux-nvdimm@...ts.01.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        ying.huang@...el.com, fengguang.wu@...el.com, bp@...e.de,
        bhelgaas@...gle.com, baiyaowei@...s.chinamobile.com, tiwai@...e.de
Subject: Re: [PATCH 2/4] mm/memory-hotplug: allow memory resources to be
 children

On 1/16/19 11:16 AM, Jerome Glisse wrote:
>> We *could* also simply truncate the existing top-level
>> "Persistent Memory" resource and take over the released address
>> space.  But, this means that if we ever decide to hot-unplug the
>> "RAM" and give it back, we need to recreate the original setup,
>> which may mean going back to the BIOS tables.
>>
>> This should have no real effect on the existing collision
>> detection because the areas that truly conflict should be marked
>> IORESOURCE_BUSY.
> 
> Still i am worrying that this might allow device private to register
> itself as a child of some un-busy resource as this patch obviously
> change the behavior of register_memory_resource()
> 
> What about instead explicitly providing parent resource to add_memory()
> and then to register_memory_resource() so if it is provided as an
> argument (!NULL) then you can __request_region(arg_res, ...) otherwise
> you keep existing code intact ?

We don't have the locking to do this, do we?  For instance, all the
locking is done below register_memory_resource(), so any previous
resource lookup is invalid by the time we get to register_memory_resource().