linux-kernel - Re: mm/memblock: export memblock_{start/end}_of

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Date:   Fri, 30 Oct 2020 10:38:42 +0200
From:   Mike Rapoport <rppt@...nel.org>
To:     Sudarshan Rajagopalan <sudaraja@...eaurora.org>
Cc:     Anshuman Khandual <anshuman.khandual@....com>,
        Mark Rutland <mark.rutland@....com>,
        David Hildenbrand <david@...hat.com>,
        Steven Price <steven.price@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Greg Kroah-Hartman <gregkh@...gle.com>,
        Pratik Patel <pratikp@...eaurora.org>
Subject: Re: mm/memblock: export memblock_{start/end}_of_DRAM

On Thu, Oct 29, 2020 at 02:29:27PM -0700, Sudarshan Rajagopalan wrote:
> Hello all,
> 
> We have a usecase where a module driver adds certain memory blocks using
> add_memory_driver_managed(), so that it can perform memory hotplug
> operations on these blocks. In general, these memory blocks aren’t something
> that gets physically added later, but is part of actual RAM that system
> booted up with. Meaning – we set the ‘mem=’ cmdline parameter to limit the
> memory and later add the remaining ones using add_memory*() variants.
> 
> The basic idea is to have driver have ownership and manage certain memory
> blocks for hotplug operations.
> 
> For the driver be able to know how much memory was limited and how much
> actually present, we take the delta of ‘bootmem physical end address’ and
> ‘memblock_end_of_DRAM’. The 'bootmem physical end address' is obtained by
> scanning the reg values in ‘memory’ DT node and determining the max
> {addr,size}. Since our driver is getting modularized, we won’t have access
> to memblock_end_of_DRAM (i.e. end address of all memory blocks after ‘mem=’
> is applied).
> 
> So checking if memblock_{start/end}_of_DRAM() symbols can be exported? Also,
> this information can be obtained by userspace by doing ‘cat /proc/iomem’ and
> greping for ‘System RAM’. So wondering if userspace can have access to such
> info, can we allow kernel module drivers have access by exporting
> memblock_{start/end}_of_DRAM().

These functions cannot be exported not because we want to hide this
information from the modules but because it is unsafe to use them.
On most architecturs these functions are __init so they are discarded
after boot anyway. Beisdes, the memory configuration known to memblock
might be not accurate in many cases as David explained in his reply.

> Or are there any other ways where a module driver can get the end address of
> system memory block?
 
What do you mean by "system memory block"? There could be a lot of
interpretations if you take into account memory hotplug, "mem=" option,
reserved and firmware memory.

I'd suggest you to describe the entire use case in more detail. Having
the complete picture would help finding a proper solution.

> Sudarshan
> 

--
Sincerely yours,
Mike.